Actually, we did evolve from reptiles (waaaaay way way back when). Mammals split off from reptiles millions of years before the dinosaurs so in a way, you could say he's a distant evolutionary cousin.

Could humans evolve to be aquatic? That's a tricky question. We aren't likely to evolve that sort of thing for a few reasons. 1. Most humans don't interact with the water enough that we would move towards an aquatic lifestyle, and 2. we don't face many of the pressures of natural selection that would drive evolutionary changes to that extreme. So I personally don't think it's likely that there will ever be aquatic humans unfortunately. BUT there are humans that do bear some aquatic adaptations. Free divers (those who can dive without the aid of dive equipment) can hold their breath far longer than most people, see reasonably well underwater, and tolerate greater water pressures. Who knows, maybe they will move towards a form with a greater affinity to water.