You rely on visual clues to place yourself on a map, so an utter lack of those (big body of water), or an unfamiliar scenery cluttered with details can cause you to lose your position. I never rely solely on GPS for navigation when paddling. I also bring maps / nautical charts and a compass with me and make sure I can position myself on the paper map at all times. I also find it easier to plan my paddling trips or next day's leg on a paper map / chart than on a small screen of a handheld GPS device.

But GPS is a nice addition. It gives me speed/distance/bearing info, and it can place me on the map when visibility is low or I'm lost otherwise. The small screen can be difficult to read in bright sunlight, also finding a useful zoom level may require constant fiddling with the unit. Total waterproofness is a must, floatability is a plus. Even if it floats, I tie it onto something, either my person or kayak.

I tend to keep the thing on all the time during paddling. I can budget for two full tour days per one set of freshly recharged NiMHs (my unit is an old Magellan Meridian Color, takes two AAs).

All this said, I did circle a couple of islands for nearly an hour once, despite having both GPS and laminated paper map. I was looking for a specific place to camp, and GPS and map were giving contradicting information. I couldn't make any sense out of it, so I just took to land. After walking around a bit I confirmed I was in the correct place and set up camp, still not quite sure of what had happened.

Next morning, I took one look at the (borrowed) paper map and noticed it had folded slightly when they fed it into laminating machine. Fold went straight accross my destination island, changing its shape and size on the map considerably. I was too tired before to notice it.

--J