Catamarans are ships that have two hulls. Though they can be employed on any ship class, they are most often employed on small, fast boats where the stability and speed is more important than survivability.
Catamarans are typically faster, more maneuverable but more vulnerable than mono-hull designs. If the opponent decides to target a single hull, then the vessel will capsize due to a greater discrepancy of flotation. However, it can actually be more powerful if they do not exploit this weakness, which is usually not the case given combat dynamics.
They are employed on every surface ship class built. However, they are often found on fast boats, frigates and even destroyers due to their lighter weight, greater maneuverability and faster speeds. Its fragility to damage is already inherit within the class, and the increase in such will not be noticeable in already fragile classes.