It actually won't do anything to you if you fall in except make your skin feel strange (like a film is on it), but the levels would allow parasites and algae to thrive. Utah Lake has been under careful observation because the phosphourous levels are causing large algal blooms to grow which could be lethal to humans and animals. It's just like a pool, you don't swim in unbalanced pools because of what can grow in it, you want to be sure only healthy organisms are living, and not the dangerous ones (especially ones that can give us, say crypto). The difference is, balancing a lake is much more difficult than a pool which simply needs various chemicals since you're not dealing with an ecosystem.