I'd vote for a small hand held GPS, like the DeLorme Earthmate units. Be aware that on a small screen, by the time you zoom in enough to see underwater features, you will be looking at an very small area, usually small enough to lose all reference to any useful landmarks. The bigger hand helds have a bigger screen, but not really big enough to be any more useful on the water. Pretty much all GPS units will track your route.

I've also owned a bunch of Garmins, but buying the maps gets spendy FAST. They have worked great for me. My current GPS is the Garmin Montana. It works great for me in my truck, motorcycle, kayak, and powerboat, but really is overkill for kayaking. The bigger screen IS useful on my powerboat.

Happy paddling!

Chuck