According to the hardcoded thread, the morale limit appears to be 63. Try setting it to max and see what happens. Of course, you must start a new game to see the effects. Existing units from savegames will not have the increased morale values. Last night three quinquiremes from Egypt with 30 morale sank five out of twenty of my old morale 8 quinquiremes. And managed to get away. However, two subsequent attacks sank the stack, whereas before it would have taken at least four.