Unless your on a map with a lot of coastal areas, they are pretty worthless. One of the main reasons is because it's nearly impossible for them to stop an invasion fleet. Once an enemy fleet gets to your shores, they can simply drop off their troops before you even have a chance to sink them. The only way to get 'em is to sink them in the open sea before they can unload. To me they are one of the few wasted units in Civ IV.