I didn't expect there to be so much interest in this subject, although I guess your experiences could actually be quite useful to most people thinking about putting a GPS on an open RIB.
Roy, I did look at the handhelds but I think that the screens are a bit small to be much use to me - without appropriate glasses, that is! The screen on the 3500c is exceptionally clear, making it easy to read on a fast moving boat. The fact that it shares the same cartography as my main navigation system is also a plus point, which reduces the risk of error when switching from one to the other.
I have since been advised that the Lowrance 3600c with an internal antenna can be retro-fitted with an external one, if it proves necessary. Problem solved!