It is strange you never saw him eat, that sounds like a red flag right there. 95% of healthy unstressed toads will eat immediately if you put appropriate sized food in front of them. I really doubt he drowned. More likely he sought out the water because something was the matter with him and eventually died there. For example toads with chytrid will often spend a great deal of time in the water. Honestly though with no more information than we have here no one will be able to venture more than a guess as to exactly what happened.