At the end of my day paddling and fishing with Adrian, I could tell that I had lost weight. My wristwatch was dangling at the base of my palm, and when I removed my waders I need to cinch my belt another notch to keep my jeans from sagging. This was water weight that I had lost, as most all of it is back today (three days later).
Our fishing involved paddling and casting for several hours over several miles. I thought that I was doing a pretty good job of keeping food and fluids in my system, but in hindsight I can see that I could have done better. I will make it a point to drink on a regular basis, even if I don't feel thirsty. That thirsty feeling is your body telling you that you are already running a fluid deficit. The hardest times to keep hydrating are when the fishing is fast and furious, and Adrian and I were catching them hand over fist for a few hours at the end. A few moments here and there to take a drink would have saved me some hydration problems the next day, where I felt hung over (which is essentially dehydration via alcohol). Lack of hydration can also lead to fatigue and loss of concentration, which can be a dangerous combination on the water.
So remember to keep fluids in you while fishing and paddling. I normally do a very good job of this, but the first time out each year tends to be a repeating cycle of neglecting this area.