So if Louis falls in the water that they became mermaids in, why doesnt he turn into one?

he would've been in the moonpool when there is a full moon over the top of the thing and he had to have been in it when that happened

That depends which episode you're talking about. Like on one epidode when the girls are moonstruck he is over the water not in it and on another episode, (Or make that two other episodes) it is not a full moon.