
At our local pool, the adults only swim is literrally just that, not a class, but laned swimming with no under 18s allowed in the pool, so if you're not a confident swimmer, I'd go for the aqua fit class, where you are unlikely to be out of your depth in the water, and it'll probably involve aerobic type & fun exercise moves, rather than just thrashing up & down lanes (which is great for fitness, but can be boring)
HTH
Claire