Most manufacturers of algaecide say it is safe to swim in a pool treated with their product after roughly 15 minutes of application. Such algae killers are designed to be effective yet safe, according to In the Swim, which makes Algaecide 50.
Robelle Algaestroy 50 is another popular algaecide that is safe to swim in 15 minutes after it has been added to a pool. As with most algaecides, it is a concentrate meant to be diluted to work effectively and safely.
In the Swim reports its algae-killing product is so mild to human beings it won't even discolor swimmers' hair.