Dunno about Clarion, but there's a company that do Sony compatible remotes, that are certainly splash proof, if not water proof.
They can also be wired with more than one (in parrallel), I have one on the dash in the cockpit, and one at the head of the bed in my boat. They work great, are illuminated and connect to the head unit via 3.5mm mini-jack.
Not sure where I got them from now, but here's a US source
And here's my Dash one, in-situ. (bottom left of passenger side dash)