The most famous fountain for coin tossing/wish making is Trevi in Rome (and I wouldn’t be surprised if the whole concept came from there). You are legally forbidden from taking money back out of it there. The moment the coin sinks into the water, it belongs to the municipality, so taking it back out constitutes theft. The municipality is allowed (and indeed forced) to clear the coins from the fountain (otherwise there would be no water left after a while) and AFAIK they donate the cash for a good cause.