M.L.
My friend's 3 1/2 year old died in a bathtub. She was older and capable as well. She hit her head on the side of the tub while playing (I think she slipped) and slumped under the water and drowned. She was only alone in the tub for about 3 minutes.
Also, some children get sleepy in the tub, I knew of another mom who walked out for a moment to answer the phone. She she walked back in, her 4 year old had fallen asleep and slipped under the water. Fortunately, the mother pulled her out before too much damaged happened.
Just isn't worth the risk at such a young age. Drowning is one of the most common deaths of children.