Ben,
This question comes up quite a bit in kayak forums. The answer is ... get the best one you can afford. Colour screens, like sounders, show more definition and offer better map detail. But at the end of the day things like (1) screen size (2) ability to upload/download maps (3) memory size for storing maps/waypoints etc. is probably more important aspects. Also consider battery power/life - this is important for long trips.
I have a Garmin GPSMap76, which suits me fine. The screen is a good size, the unit is waterproof and it floats. It runs on AA batteries, which is good for long trips - as I can simply carry some spares and change over when needed. It is easy to use.
Cheers,
Pete