This is one of those rare moments where my experience outside of pool might apply.
A lot of people like to think "carb up the night before" but the truth is that most of those nutrients are either burned up right away as simple sugars or aren't really processed and available until the end of the following day if not later. (digestion miles may vary)
Honestly, your last shot at getting actual nutrients into your system is breakfast (possibly an early lunch) the day before. What you eat for dinner should be more about feeling full enough to sleep but not so full you're bloated. Try not to eat anything new or complicated.
Physical activity will consume a combination of glycogen and fat at a ratio based on the level of activity. Sprinting burns almost all glycogen and no fat. Walking slowly burns a higher ratio of fat. So, really the concept of carbing up for pool probably doesn't make too much sense as it relates to the physical side.
However, your brain functions best on glycogen. It will burn that for it's activity first and fall back to the fat only as necessary. Have you ever felt your mind become fuzzy when you haven't eaten in awhile? That's most likely your brain trying to work with an insufficient glycogen store.
My recommendation is to eat something somewhat filling with two servings of protein, one serving of grain, one fruit and one vegetable the day before somewhat early in the day. That will process and be available to your system during the event. Eat a balanced dinner but the point there will be more about ensuring good sleep. To help keep your mind alert and active I'd recommend staying just a hint above hungry for as much of the day as possible. To keep your energy up, take with you some kind of carb snack you can consume during the event at regular intervals. (clif bars, fruit, raisins, etc.) You want something relatively easy to convert to glycogen but not sweet and sugary to avoid the resulting high and low. I would avoid eating any big meals during the event. You'll end up converting your body's attention to your stomach and the drain on your energy levels from digestion will make you tired. If you're hungry during the event but don't feel you need more carbs I recommend almonds or cashews as a snack. They will fill you up without creating a carb or sugar high.
On the other hand, buffalo wings taste better so feel free to throw out all of this advice. It's just leftover from a few too many years of endurance events.