My selections would be a Navico Axis VHF (now made by Simrad) and a Garmin 12 GPS.
The Navico is probably the most expensive hand held VHF available, but it is fully waterproof and very rugged -- and it performs well. Simrad also now make a smaller waterproof VHF which is probably good too. Icom also make a waterproof VHF that is worth a look.
For use on a RIB I personally wouldn't buy a handheld VHF that isn't waterproof. Plenty of people will disagree with me though, and swear by the waterproof plastic bag enclosures that you can buy for a few pounds.
The Garmin hand held GPS is easy to use, has a good display and is waterproof. It's relatively cheap too at about £100.