Hi

Are you trying to simply submerge the part ?( = it's inside the tank) -- or-- are you trying to use the part to stop water flow? (= it's a stopper or gasket).

Nylon is a really bad thing to use in full time contact with water. It soaks up water and swells or it dries out and shrinks. Ultimately with enough water around it will crack and degrade. ABS is a much better option.

If you want water tight, a filament printer probably isn't your best choice. Even with a full infill, there are still paths though the print. If pressure is involved ... not a good thing.

Bob