A caravel with one extra move as your only UU is terrible though. So no buffs for fighting at all. Worse UU out of all 43, I would say. Even worse that the Korean Turtle ship I think -- which is also terrible -- but at least has a high combat strength. I guess the extra move is good for the main role Caravels have, unfogging the map, but at the cost of never having an advantage in fight?
I think the UU is pretty decent, and it's certainly not the worst in the game, others come to mind like Slinger and the Naresuan elephant. The turtle ship is also a situational bad unit.
Caravels are not really supposed to be used in naval combat (this is why they have the withdraw before melee ability). They are more for exploration and the 1 extra movement for Nau is a good bonus. Plus they also have the cargo ability, which gives you gold AND xp. If you start with armory and get some XP towards that 3rd promotion they can make really good ironclads later on. I wouldn't say that they have no advantage in fight.