What is your budget?
What kind of water are you kayaking on? (eg: ocean, lake, river, what area?)
Are you going to use it for anything but kayaking?
No real point in going high end unless you are planning to load maps into it. Loading in maps of water requires purchasing map cd's and that decent maps are made of the area where you will be. That said, I love the mapping GPS units that I've had.
High end units offer mapping, routing, color, and some bells and whistles most people don't use. Can be very useful, but you'll pay for it.
If you want to keep it cheap and simple, get a $100 unit and you'll get everything done but the fancy stuff. The other nice thing is that watching $100 dissapear into the gloomy depths is a lot easier than watching $400-600 go. As long as it is WAAS enabled you'll find no difference in accuracy.
Check the Garmin website
www.garmin.com and start comparing features. Drop a note anytime if you have specific questions.
FWIW, the last GPS handheld that I bought I got from
www.tvnav.com I found them very knowledgable, they had great customer service, and they had the best price.
Hope that was helpful even though it was mighty vague.
Tol