3DPrinting

My approach would be to combine pretty much what everyone else has said, and add annealing to the process:

1. More walls, higher temp, bricklayers (the usual wank when you're more into 3d printing tech than design). Test if watertight and proceed to #2 when it obviously isn't
2. Can I just buy the container?
3. No? Then annealing could be a solution. Take the print from #1 and put it in a baking tray. Fill out every bit of air with sand to support the print. Bake the print at just above the glass temp for the filament for a enough time for the sand to get up to temp. Turn off oven and let the entire thing cool down slowly. Test the dimensions and watertightness.
4. Realize that I didn't do a good job of supporting the structure and I spent too much material printing extra walls. Others have suggested coating the print, which will add material in a somewhat unpredictable manner. Is it acceptable in view of my tolerances, can I alter the design to make it acceptable? Print with minimal material and coat with epoxy.
5. Get tired of the project and start comfort printing cute stuff for people you like...

Whatever the smartest person here says, and since they're likely thinking of using it as a water container, multiply it by the depth .

Water will be pushing the air inside, seeping in hard, and buckling your surfaces. My understanding is the best counter to that is submersing your (electronics? ) in oil which is incompressible and not conductive. At which point a cheap Tupperware from Dollar store would also do the job.

So what you're thinking... Then coat it with epoxy.

Watertight and waterproof are not quite the same thing.

Almost all 3D printable materials are waterproof, in that they will not dissolve in water. (With the exception of, e.g., PVA which is sometimes used as a dissolvable support material.) I realize this is not the intent of your question, but a lot of people seem to get it twisted about various polymers absorbing moisture/being hygroscopic/becoming "wet" and therefore believe that they literally melt or soften in water over time or something. This is not the case.

3D prints can be made watertight but it does not necessarily follow that all of them are by default. This will be dependent on your print settings and, to a certain extent, your print material. Some materials are more isotropic than others and the layer lines stick together more readily without gaps. TPU leaps to mind, which can be made extremely watertight very easily.

Use a lot of walls -- another poster recommended 4, that's probably a good place to start. Don't forget to increase your top and bottom layer counts as well. You may need more top or bottom layers than walls, because your layers are probably thinner in the Z axis than your nozzle extrudes in X and Y. If dimensional clearance is not an issue and in your case it seems it isn't, consider increasing your extrusion multiplier slightly in the walls as well, to ensure that material is squished into any potential gaps. Avoid sharp corners or tiny points on your model, which upon slicing may be incompletely filled. Avoid long unsupported bridges as well, because the couple of layers where these inevitably sag will wind up non-solid. If possible, make the outer shell of your model an exact multiple of your wall extrusion thickness so your slicer will not have to guess at any areas and try to fill them with tiny points or similar. If you play back your slicer's preview of a single layer you'll see what I mean.

If you really want to employ the nuclear option, instruct your slicer to iron every single layer. This will make your print take forever, but each individual later will be extremely authoritatively bonded together in the X and Y axes, with no gaps.

If failure is not an option, coat your object after completion with Flex Seal or Plasti-Dip or something.

Sometimes a regular manufactured item is better than a 3D printed object. Just use an old tupperware container

The guys from Prusa Research have a lenghty blog post about watertight 3d-prints: https://blog.prusa3d.com/watertight-3d-printing-part-2_53638/ This might be a good source for tipps and tricks!

I made a small planter and I sprayed it with some sort of clear sealant for extra measure and it has never leaked.

For you, under pressure might leak after a little while, but you could put some sort of sealant on it to have more faith in it. It depends how long you want it under water, but it should be at least a little water-proof by itself.

Sorry no better experience than that

If it's non-critical, with a bit of work it can be made 'waterproof', at least at 1atm. But realistically, unless you need a specific shape, it's going to be easier and more reliable to just use an off-the-shelf case.

I've played around with using PLA to make plant pots on an old Ender 3. It wasn't quite waterproof, but pretty close. If you wanted a perfect seal for underwater shenanigans, I'd probably experiment with different wall thickness settings, and maybe temperatures/fan cooling settings. You want to try and minimise the tiny air pockets between layer and filament drawing lines. (Resin 3D printers are probably better at this tbh.)

I had reasonable success using a hot air gun to melt the outer layer of PLA to make it perfectly smooth, but this can deform your object if it heats up too much, so I'm guessing very hot temperatures and less contact time would be the ideal setting.

You could also try smearing Vaseline or resin coating the outside of the object.

I think with a sufficient wall-thickness your box would be waterproof. If I were you I would test it first by printing a small box like you described you would, and put a piece of paper with ink writing inside. Then submerge it into water for a few hours. After that, let it dry thoroughly and saw it open to see if the paper inside has gotten wet or the ink writing has been smeared/diluted.

From experience printing a bunch of different planters with varying settings and printers, without post-processing/sealing the answer is "not reliably". You can increase your chances by using many perimeters and surface layers, high heat, and thicc extrusions, but you are still just stacking swiss cheese slices on top of each other hoping the holes don't line up.

With days submerged, even the tiniest of defects means water can and will seep in.

3d extrusion prints have microscopic holes inside, it's porous. I wouldn't trust it depending on the application. You can seal the outside and they also sell additives to lessen the effect (here's a video about it: https://m.youtube.com/watch?v=Q8x-mjjT8j4) but casting resin might be better (haven't tried). Resin printing doesn't have these problems from what I understand but ou can't really stop the print midway.

Fiberglass, if the object isnt complicated to make, would be the best option in terms of quality. Bending and cutting pvc sheets into a box then soldering is also an option. Soldering pvc and rolling fiberglass isn't really difficult but they both require specialized tools/materials. Getting pvc pipes and soldering end caps would probably be the easiest.

How robust you need to make it largely depends on pressure and depth.

Of course it can be.

Will it be on your setup though?

Bump temperature to make better adhesion, lower the speed. Then try it out!

Also if you can't make it perfectly watertight, check out why and try to remedy it. Maybe post printing! You can put glue, paint, depending on why it leaks, chuck it in the owen to melt it a bit...

But first of all, try it out!

Good luck!

How big of equipment? A ziplock freezer bag might do the trick for you, or if you're fancy, vacuum seal it. I trust that to sous vide meat for days, at elevated temps even.

What is the sensor sensing?

It's the number of walls you print that will determine just how long the box will hold the water in or out. I find 4 walls to be about as water tight as I need. But I'm not sealing anything valuable either.

Edit to add: This would be an excellent need for that brick layer printing to be used. The offset layers would provide far better sealing than the standard layer method we commonly use.

I do not 3D print, but i do have some basic knowledge of it? I am not a ~~lawyer~~ printer, YMMV.

I have heard that some materials can be smoothed over using acetone vapour; what about wiping the finished print to smooth over open pores on the surface of a print?

Alternately, since it's not a flexible material what about applying something like 3M never wet?

In my experience, TPU is fairly easy to make watertight. You have to start with good and dry material so it prints as smoothly as possible. It floats pretty easily though, so you might need to print it solid or with high infill.

A couple of months ago I printed a bird bath out of PETG which has been watertight so far. I used three outer layers.

Haven't tried it myself, but have heard in passing that they are generally not waterproof. Might be different for different materials or print orientations though?

Or you can do some post processing, add a coating, or vapor smooth?