Even looking at high end P25 mobiles, you will find that they are IP-55 or IP56 ratings, which are good for driven rain, etc. Waterproof is a very nebulous term, as there are different standards. Submersible, water jet, driven rain, "splash resistant" etc are all some of the terms used. IP ratings, Mil-810, etc. are all things you'd need to consider.
If your RHI's have a center console, careful placement will keep any direct splashing off them, but they'll still get damp. A lot of the higher end radios have control heads that get installed on fire truck pump panels, so they'll take some abuse, but not sure how well they'd stand up to salt water.
Since these higher end radios get really expensive, looking for a waterproof enclosure might be a better option. It, at least, would give you more options in radios. One thing to consider would be how often the radios would need their controls changed. If changing channels, etc was a rare event, keeping the radio protected, but having a water resistant speaker and microphone might be a good option. If frequent channel changes, etc are likely, then this will be an issue.
To do it by "the book" you'd need a radio that not only has FCC part 80 certification for the marine VHF channels, but also part 90 for the LMR channels. It isn't difficult to find a radio that will do this, legally.
Another option might be to find a radio that uses a hand held control head. Both Motorola and Kenwood make such radios. While not a permanent solution, it would reduce the amount of hardware that would need to be replaced when it gets damaged.
Some other options:
Get a marine VHF "mobile" type radio, as they would be more suitable, then use a submersible rated VHF hand held for the LMR stuff.
Get basic radios and seal them up the best you can, and just count on frequent replacement.
I've used Motorola CDM radios with a remote head set up on off road vehicles where they were exposed to vibration, dust and occasional rain/splash environments. So far they've all held up well. Most modern LMR radios are actually sealed up pretty well. I wouldn't say "waterproof", but they are pretty good.
Last time I looked, the USCG was using Motorola XTL, or at least that is what I remember. They tend to protect their stuff in the cabin, but likely if they are sticking with them, they are standing up well.
Should be interesting to see what others post, this is an interesting question.
