the eye mustn't be attached,but the whole body must be one piece. So you just have to combine the eye with the body and be sure that the whole body is watertight.
The same for the mouth. if there is a hole the mesh isn't watertight.
edit: if you want to attach sth. you have to do this:
1. combine the eye and the body
2. select 2 vertexes which are going to be one vertex
3. go to mesh (or edit polygons in another maya version) then merge (or merge vertexes)
that's it, good luck and have fun
(it's my 100th post, haha)