Well, for starters, you were having so much fun on the water that the basics kind of slipped away. The rules are that you drink before you're thirsty and eat before you're hungry. Otherwise, just as you're experiencing now, you'll bonk. Crash. Melt on the beach.
In most cases, with moderate exercise and moderate weather, you really want to drink about a liter of water an hour. If you think that's too much, and you have urges to find a private bush on the beach, try a little experiment. Take a bathroom scale along with you and just before you launch weigh yourself. When you take out at the end of the day weigh yourself again. If you're down a pound, or two, or three, what you're measuring is water loss.
Prevention is the best cure, so to prevent that washed-out feeling, start taking in fluids well before you launch. Fluids don't include coffee, because that will leach the liquids right out of you. Water's good, fruit juices work, sports drinks fill the bill. Some athletes add a bit of sugar and a bit of salt to their water. Also, nibble on sports bars or fruit as you cruise along.
Those fluids and electrolytes will help maintain your cardiac output and stabilize your body temperature while you're paddling - big-time, and with major plusses. You'll feel better, you'll paddle more efficiently with less apparent effort, and you'll think better.
On the food side, carbohydrates should be eaten before, during and after exercise. Sports folks believe that you replace glycogen stores by consuming carbohydrates. The hormone regulating glycogen is insulin, which increases the transport of glucose from the bloodstream into the muscle masses and fires up the enzyme that converts the glucose to glycogen. Insulin also directs amino acids into the muscle masses to kick off the rebuilding and repair process. It appears the best mix you can swallow is about four grams of carbohydrates for each gram of protein.
As you slump on the beach, you notice your muscles hurt. This is a surprise? As you paddle, free radicals build up in your muscles and may well be the main cause for cell membrane damage. You may be able to relieve this oxidative stress by popping antioxidants such as Vitamins C and E.
Repair of your exercise-worn muscles goes on all the time, accelerating from when you start to paddle/exercise, and continuing after you hit the beach at the end of the day. It could be that adding a bit more glutamine to your diet can help. Glutamine is a natural amino acid found in the protein in plants and animals, and by upping your protein intake you'll help repair muscle cells.
When you're trashed at the sunset end of a paddling day, maybe it is your body telling you to mind your food and drink.