There is no way to know if it would or would not have happened if lifeguards were there.
Exactly, last summer our brand new spanking water park (well, not a big grand thing..pool, zero depth entry, water play structures, deep end, slides, diving boards), had a drowning, and there were lifeguards on duty of a 3 or 4 or 5 year old child.
A drowning can happen in as little as one (1) inch of water.