Early on I used GPS to see the speed at which I was paddling, because there is no better way to see if you are using your torso than the speed you are going. You will paddle faster with significantly less work using your torso and legs than you will with your arms. That said, it's been a long time since I have done this, but for new paddlers it is an effective method.
Generally I use a gps to make waypoints at important locations. A put in/take out. The start of a crossing. a good campsite. But I tend to make a waypoint, and then turn off my GPS. This saves batteries, and also helps me keep my focus where it needs to be. On my paddling and the environment around me.
I want to relate two stories about the use of GPS. the first is a hiking story.
I was hiking in an area with terrible maps, only slightly less terrible than the trail markings. I had made a waypoint at the trail head, As well as a swimming hole I thought I might go back to, and a stream crossing. I soon realized that I was seeing things in the real world that weren't lining up with the map, and things on the map didn't match the real world. I started to get that little feeling in the pit of my stomach that things weren't right. So I literally sat down in the middle of the trail, spread out a map, and based on where I thought I was, figured a bearing to where the car was, and also calculated a distance. When I was done, and thought I had a pretty good understanding of what was going on, I took out a GPS and told it to 'goto' the car at the trail head. This gave me bearing and a distance. I was off by a few degrees, and the distance was close enough. It told me that I knew where I was. It confirmed the map work, and let me relax a little. This is a good use for GPS, in my opinion.
The second story is a paddling story. At the end of a week of paddling, along a group of barrier islands with a friend, we decided to skip our last campsite, and cross a sound back to our car and put in. It would only add 3 or 4 miles to our days journey, but we were tired, and wanted a real meal. As we were getting close to our crossing the weather changed. We got an unexpected squall, and we hunkered at a small island at the beginning of the crossing. We waited about 45 minutes, once the squall passed - even though we still had some wind, and swell - we realized that this was our opportunity to make the crossing and get to safety. We picked a landmark on the far shore - slightly upwind of our destination - agreed on a compass heading, as we both had deck compasses and headed off. We had a fairly big swell, and my biggest fear was a power boat, trying to get home in lull after the squall - as we were - not seeing us, and running us over. I told my friend to keep his head 'on a swivel' so we could see everything happening around us. The last thing we did was this. He had a waypoint for the car, and I created one where we were. He told his gps to 'goto' the car waypoint, and I told mine to 'goto' the point I had just created at the start of our crossing. So his GPS would tell us distance, speed and bearing in relation to where we were going. My GPS would tell us distance speed, and bearing to where we were coming from. At any point we knew - if we had to retreat - where safety was closer. In front or behind. We knew how long the crossing would take because we were monitoring our speed. But here is the most important piece of information that we got from the gps units.
Kayaks are small craft and very susceptible to being pushed in a direction other than that intended. It is a concern on crossings that while we paddle forward, a current may push us sideways. With the two GPS units telling us bearings to our way points, we could see how we were moving in relation to those waypoints. We could tell if we were moving sideways instead of forward - though sometimes it can be both.
The only time I am a fan of using gps with the unit on for a long period of time and in front of the paddler where it may cause us to lose focus on our surroundings is on crossings. In part because it is good to know how fast I am going, and how long the crossing will take, but it is that potential for side to side movement that I am most concerned about, because without a gps, or careful sightings with a compass - which is not easy to do in the middle of a crossing - You may not be aware of that unintended direction.