Nothing lethal. The whole fear factor about samonella in iguanas is often times overplayed. Yes they have salmonella.. so do you and I, its more or less a matter of what bacterial strain it is and the bacteria count. A well taken care of iguana is typically going to have a safe, balanced bacteria load. The chemicals in the pool would probably kill the bacteria I would guess. I am sure birds have dropped a million turds in that water, and like iguanas they too have salmonella like any other critter.
Water DOES however make them poop.. 3 different stages in their poop... A clear fluid substance, an egg yolk like substance and a brown turd. I am sure the filter would eventually clear it out, but thats a gross factor to me.