They require Map Making. They can carry only one or two land units and with maximum "speed" of 2.5 and are restricted to "coast" (shallow water) tiles and ports. Defensive value is 3 (with no offensive strength) and standard cost is 20 material.
Valuable for exploration if your world is less than 90% land though less valuable on a small world.. Valuable for finding exploitable fish that are just out of range of early land unit view.
The standard AI often sends out flotillas of them, very inefficient for exploration but often good for blocking other nations' boats.