A certain amount of the boat has to sink in order to stay level. If the bottom of the boat doesn't sink, it's balancing on a knife's edge - and topples over.
Right; I suspect a raft or any other flat bottomed boat would be fine.
Which only applies to some kinds of boats. Many other boats won't have the same issue.
Yes.
Some boats would work find, depending on the geometry. A pontoon boat, for example.