Well first off...can't comment too much on naval battles as only one of the 2 maps even has water, and the AI won't build ships (other than fishing ships). However...I've played with them a bit, and they are very cool.
First off, the ships are HUGE! Additionally, they do not use up population slots (aside from fishing boats). All can also act as a transport (except fishing boats). To balance the no pop slots, they made the boats very expensive, and limit the number of them you can have of each.
Galleons are probably the coolest as when they are on the shore, you can use them to train any barracks or stable units, as well as grenadiers I believe (a seige factory unit). They will be very popular on water maps as they are a mobile forward base.
Monitors are huge ships that have long distance (100) bombardment attacks that do 10x damage to buildings.
Frigates, Caravels, and Man O Wars are the other ship types. I will try and get you some pics of them. They do look really cool.