In answer to the above question, Stugeron is an over the counter motion sickness treatment available in the UK (where I live). The active ingredient is cinnarizine. I'm afraid I don't know what the US branding would be. I find it very effective indeed.
One other thing to consider - I've never swum in a lake before; only ever in the sea where I can see the bottom. I was practising swimming in a straight line the other day by closing my eyes in the pool and even after a second or two I suddenly felt very dizzy, so that's another thing to consider (and there's no solution to that I'm afraid other than swimming head up).
The most common cause for dizziness is cold water though, and if you're right and the lake is 80 degrees (about 27C?), then it can't be that. You can tell that sort of dizziness because there's no interim "woozy" stage - it just goes straight into crazy world spinning vomit territory