How do I attack the water provinces? (and other q's)

[ragnar]

Okay I really still feel a bit clueless here... I bought the game and am now halfway through my first game.

Questions for you experienced guys:

1) There is this annoying god that owns all the provinces in the water AROUND my land. He pretty much can invade any coast country he wants to - and I can't attack him. What do I do?

2) What are the best units for storming a castle? Once I successfully siege it, I see I can storm it, but it's always tough.

3) Okay VERY dumb question here. Is the only purpose of Dominion to give your pretender god more hitpoints and help their spells work better? Because if that is it, it doesn't seem like such a big deal... I'd rather have 20 heavy cavalry than a commander with slightly more potency - right? Maybe I have something wrong here.