What sort of depth are you looking for.
I use a simple supermarket sandwich container for the enclosure on my RC airboat, but it is mainly for splash and occasional inch of submersion when I capsize it.
If you need depth things get interesting. Some friends of mine did a senior project building a submarine. They had a serious hull with wires going through silicone sealed holes, but they found that at depth water would get in by tunneling between the wire and the insulation. They thought they had covered everything, but at depth things you would have never thought of tend to come up.