People can drown in bathtubs from alcohol intoxication. A heroin OD can result in the same but there's no guarantee that you'll pass out, slip under the water and die. I don't see anything wrong with being in a bathtub and then ODing though. A warm bath can be a comfortable and relaxing place to die in.
And if you do manage to slip under the water, if you have zero/low tolerance to H and the H is strong, you probably won't even know what's happening or be too out of it to save yourself.