This is my say. The Iberians should be able to merely stock up their merchant vessels and such with soldiers. As their javelin throwers would be able to hurl flaming spears, enemy ships actually could be destroyed. The thing that should be understood though, is the kind of strategy the Iberians would be made for. As they have only a bad navy in the last few ages, they need to rush their opponents on a sea map. Likewise, instead of going to the unnecessary trouble of making weaker cultures, it could be possible to perhaps merely make trade agreements with your rivals for units.