Early in the game, none of your officers have much combat ability and neither do you, so the only way any of you is going to build up that ability is by going on land and taking chances. So, if you're planning on putting a carpenter in harm's way later, get one early in the game and take him down all the dungeons. Alternatively, once you have three officers with high Melee skill and all the combat perks, replace one of them with the carpenter, then look after him and protect him if you get into a fight - eventually he'll build up his combat skills. This, in fact, was why I suggested that Melee XP be shared among everyone in your shore party regardless of officer type, some time ago - so that you could train more officers in combat.