Old Nalgene bottles could be recruited for underwater enclosures for microcontrollers. They are tough bottles and have big sturdy lids which could host robust conduits for sensor or antenna cables to pierce the hull.
I ordered some lamp pipe parts from grandbrass.com to try making waterproof cable penetrators in Nalgene bottle lids. I bought both brass and steel versions of the threaded pipe nipples and nuts. The steel ones are not stainless steel so the brass will survive better underwater, but brass contains lead, so is a bit nasty.
I also ordered some stainless steel washers and neoprene washers from McMaster Carr and had some copper washers from a boatbuilding project. Everything is for 3/8” diameter bolt size, but the threads are non-standard (27 threads/inch). So standard nuts don't work.
The classic Nalgene bottle has a lid leash connector on the lid which I cut off, ground smooth, and sanded.
Then I drilled a 3/8" hole for the threaded pipe nipple.
The pipe was turned into the hole threading the plastic and making a tight seal all by itself.
Nalgene washers and metal washers were squeezed tightly by the nuts on either side of the lid.
The Nalgene washer is a tight fit on the nipple, and when it is squashed it expands against the threaded nipple making a good seal. I don't know if adding some grease will make the seal more waterproof or just more slippery.
The copper washers had to be ground down on one side to fit under the lid. There is enough room to have three of these cable penetrators on one lid.
Several wires can fit through the nipples.
The final step is to pot the wires the entire length of the nipple. The best stuff for this is apparently two part epoxy. Ed Mallon seems to have good success with Loctite E-30CL. I wonder whether it's worth trying one of these silicone sealants which are a lot cheaper and maybe easier to use.
I had to buy multiples of all the parts to justify the shipping costs, so I have some extras. With shipping the brass version cost me $5.00 per set, but the steel version was under $2.00 per set. If anyone is interested maybe I can put them at the KAPtery. Shipping will add a couple more bucks, so the price won't be very different from purpose built cable penetrators.