I fitted a Navman 5500i colour chartplotter to my RIB earlier this year and love it. The C-Map cartography is superb and the unit has performed faultlessly so far despite having had several good soakings. The technical support guys at the importer, Plastimo UK, in Chandlers Ford were also very helpful when I rang them with a few questions about wiring etc.
Navman plotters were recommended to me by a friend in Scotland who's a Fisheries Board manager and they've been using the Navman 5500i on their RIB for three years without a hint of a problem or any sign of water ingress etc. Having said all that, Garmin plotters have an excellent reputation and the Garmin back-up is of course legendary. Some people reckon that C-Map cartography is better than Blue Chart, however, so you pays your money (less in the case of Navman
), and takes your choice.