I agree with everything Ronbo has stated except the "you don't need a GPS" statement...
You are so right Soundfisher. Whether alone or in a group, if someone has cardiac problems or some other emergency, you get on the cell phone and what will emergency people ask? What is your location? Umm, I'm in the Gulf of Mexico, but I can see land. Lets see, the compass says Northeast to whatever that building is in the distance. Those without a GPS are in trouble in an emergency. $100 dollars is a cheap form of insurance.
A GPS will give you live time speed. By knowing your average speed, you can determine what distances you are capable of. How far you can go in one direction before having to turn around to get back before dark?
It is good to know what your normal kayaking speed is. Say a local kayaker is leading a kayak trip and he/she indicates the intended speed is 4.5 mph. If a novice kayaker who has no idea what speed they paddle or pedals shows up, guess who will be slowing up the group or will be left behind out in the middle of nowhere?
A GPS used in conjunction with the Pitot Tube will give you real time data to let you know whether you are flowing with or against the current. If the Pitot Tube says you are moving at 4.0 but your GPS only shows 2.5 mph movement, you're fighting a 1.5 mph current. Currents in rivers are easy to see, but when in open waters, currents and your visible speed are more elusive.
You can track your trips using some brands of GPS and download those trips to Terrain Navigator or Google Earth Plus. You can also plan a route and upload that route to your GPS so you stay on track. I've seen leaders of club trips paddle right past the entrance to the tributary they planned to explore only to stop 30 minutes later because they knew something wasn't right. They could have spent 15 minutes planning and uploading the trip route to the GPS to avoid getting lost.
Never get lost with a GPS. I have had to rely on a GPS when exploring creeks with many branches and turns. (We've got about 400 sq miles of waterways around this area.) Can't remember which way you turned because the turns all look similar and because of the clouds, you are not ever sure which way is north? Look at your GPS - all you have to do is follow the track backwards.
While a compass is good, I wouldn't want to rely on one in the fog. It is basically a guessing game when using a compass in fog. Go out in the fog with a GPS and using waypoints, you'll be able to go exactly where you expect to go.
The GPS is not a toy, but a valuable navigational tool. Like American Express, don't leave home without it.