How to seal electronics against water, according to my long-ago sailing experience:
1) Spray conformal coating on it.
2) Drench it in potting compound.
3) Put it in a clamshell with tight rubber gaskets.
4) Drill a hole in it for the water to come out through.
Really, water will get EVERYWHERE. Even if you think your cables are tightly sealed by the epoxy and sealant, water will work its way in the cracks through capillary action.
The best way to work it is to use multiple layers of boxing, that have enough gaps to let the water out, but are boxed such that a spray won't let the water in past the first box.
If you need to fully submerse it, build a submarine :-)
(Actually there are some people building battle ships in balsa that use steel balls for ammo, and they just pot all their electronics in similar silicone sealant, and it works for shorter periods of submersion just fine.)