Aliscafi (i.e. hydrofoils) and most kinds of smaller fast and "superfast" boats almost never have "outdoor decks". The main reason for this lack is safety, plus the fact that one always gets wet through the combination of speed and waves. Actually, given the way these fast vessels are designed there is simply no space to accomodate an open deck. On the other hand, "conventional" smaller ferries almost always have "outdoor decks"; they are, though, about 50% slower and, certainly, cheaper.