I find researching the Ironclads tech useful. The AIs usually don't research it immediately, so it can be traded around. There is a chance of getting a SGL, too (it happened to me once or twice when playing a non-scientific civ). Ironclads are slow, but they are good in bombarding and give a better defence for Galleons than Frigates (a veteran Galleon is always attacked first when it is protected only by veteran Frigates). And, as it was mentioned, they upgrade to Destroyers.