ur fleet or fleets of ships are the same coloured bases as the army ur commanding they should b in around the areas / lands u own . as far as i know they can only attack enemy ships by right click and hold them above the enemy till they fade in colour then let go , it will ask if you want to attack .

you can't use fleets to actually attack islands, just make several ships EX: build ships in Denmark, and make a unbroken chain of them to the mainland of britain, if there aren't any warships of your enemy, then you should be able to take trrops and land them.