Just water (instead of gel or gatorade)

OK, so I've learned that I can down a couple of cheeseburgers and a couple of beers and go out and swim 4K no problem -- even feels good. But I'm trying to find the "sensible" solution for either a 10K or longer swim or the shorter 3K-5K days on extremely hot days. Last year I swam a 3K in 90+ weather after driving two hours in my non-A/C Jeep to get to the race. Thinking I should hydrate, I made sure I had plenty of gatorade before starting the swim. I felt nauseous and my legs turned to rubber 1K in. On the cooler 3K-5K races, I do nothing but drink some water before and have always done fine as long as I have the beer waiting at the end. But by the end of 5K I always feel like I need something more if I want to go any longer than that. In prepping for eventually entering a 10K, I began to experiment with gels. I've tried two. 10 minutes after eating gu or PowerGel, I got light-headed and legs turned to rubber. I researched previous threads and saw that some folk just drink water for anything less than 8 miles or so. Anyone think differently? Any alternatives? I'm not sure I want to stuff cheeseburgers and beer in my suit. . . but they might work for the 10K postal.