HIV is nothing to worry about. I doubt that the virus could survive more than a few seconds in your average pool, lake or ocean. This would be something, however, I'd expect a buddy to mention before the dive.
If you want to scare yourself, take a drop of ocean or lake water, put it on a slide, and take a peek in a microscope. There's more wiggly critters in a single drop than all the animals in the all the zoos in all the world. That's what's going inside every time you put your reg in your mouth after it's been in the water.
When you estimate the visibility, stop and think what you're looking at. It's all the micro- and macroscopic crud suspended in the water. If people weren't already equipped with effective immune systems designed to deal with it, we'd all get some kind of raging infection every time we went in the water.
