Voltar - are you sure he wasn't a veteran, and yours not?
However, I have deliberately balanced the game to marginally favour the defender. This is because the advantage was with the defender in lines-of-battle at sea.
If the attacker was much better quality (like the English usually were) they could get away with a frontal attack. They would soon regret it if their opponents were as steady and as good at gunnery as they were!
In standard Civ, the advantage was always with the naval attacker. You would sneak out of your port, zap the defender, then scuttle back in to repair. Not so easy now, and the English AI makes a good job of defending the English channel usually, which it has to as the French have a much stronger army.
It is possible to win naval attacks - gang up in larger numbers, and attack smaller ships with heavier ones. To attack a three-decker, you need at least two warships, preferably both three-deckers. And try to make sure they are veteran by practicing on smaller ships first!
However, the Great Austrian navy SHOULD struggle to beat the French!