Might be worth considering the Standard Horizon HX851e, waterproof floating DSC handheld, with a waterproof fist mic. The charging base can be screwed down, has a 12v feed, and even has NMEA out for communicating to any other electronics you might have on board.
A neat package that gives you all the functionality you want, with the added benefit of being able to remove it for security. I've got two S/H DSC Handhelds, and they're great bits of kit.
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