we might see a Viking Longboat as an UU.
perhaps diversitfy the modern naval units such as perhaps add an AEGIS Cruiser, but that depends on revamping the air warfare element. the thing is that its not just more units, the whole naval warfare thing and the ships capabilities needs huge improvement.
I don't know about early naval ships as there weren't that many types. For middle ages, we have the Galleon and the Frigates, which are classics, maybe a small but fast (with a free flanking promotion) ship which could be a Schooner or something, and there are also those great big warships.
IMO they should improve naval and air warfare first and then think about new ship units.
also, those transports should carry more units, in Civ2 and Civ3, those transports could carry up to 6 units I think.