Reed switches is what's used in encapsulated electronics (even in heart pacers they use a reed switch). Cheaper and far more reliable than Hall Effect Switches (and don't need power to operate).
I was looking for high power reeds to see if it is possible for the reed to disconnect the battery - but none seem to be over 5A carry currents. There is a safety reed from telemechanique that seems to be 100A rating - but it is big and red and over 100$. Maybe it has a relay inside?
So it can control a very low Ron FET, or it may be possible to use a reed switch as a "use once" fuse (replaced every time it falls into the ocean). Even then, having that type of current through a 5A rated switch is too much (even for me).
Sealing techniques:
Another "sealing" technique, one favored by linesmen, is vaving an inverted "can" or "box", open only at the bottom. Sensitive electronics is inserted from the bottom, and cables are arranged to be "scattered" so that wicking of water up cables is limited (or coated in that yucky hydrophobic grease). In your case, that would mean the copter should only crash upright (which I don't know if doable) and should never tip inverted.
The best sealing technique, that one we used most often in the navy, was waterproof aluminum boxes that had waterproof glands. We used Bendix/Amp really expensive glands, but there are cheap plastic ones available. Now the key to using such a box is that the gland has an Oring that is a perfect fit to the cable. Another Oring sealed the gland to the box (or it was a flat profile O-Ring). Sometimes, the connector itself was the gland (REALLY EXPENSIVE!!! - but may find NOS on ebay). We used specified lubrication to ensure the Oring was seated properly.
The trick is the cable. The cable has to be round. Not fairly round - absolutely round. It has to have a surface that is sealable like PVC, and more importantly, needs to be sealed at both terminated sides - or you must use a cable that sealed against water ingress. The latter cables are injected with hydrophobic grease that is substantially more annoying than silicon or petroleum grease. Otherwise water would wick up the cable into the box.
As examples:
For systems like active sonar, the cables were a core of individual transducer HV pairs, each core sealed. The area between pairs was fabric strands with the yucky grease. this core was overmolded in real tough thermoplastic. Above that two alternating direction layers of steel cabling was wrapped (to carry the tons of weight of the transducer array). These were carefully terminated to large load carrying terminations.
Smaller side scan sonars use a substantially smaller version of same, and newer generations put a lot of the electronics in the transducer. In an active SONAR that is impossible since the amplifiers are huge.
I don't think any of this is necessary. Just conformal coating and getting rid of the potential energy of the battery upon impact should be enough. BTW - What about the motors? If salt water gets into the motors I assume that would destroy the bearings?