Why "upgrade" in the first place? I mean....I suppose you can if you want...but why spend money you don't have to? A fully grown female BP will do just fine with a little deli-cup for fresh water. They don't need swimming pools.
That being said...as for the question of drowning, I wouldn't put something that size in with a little baby BP...but so long as there is no lid or other obstruction to them finding their way out, I dont think there's too much risk of drowning in their water bowl.