The province you're invading from must have a port

You need an unbroken chain of sea provinces connecting your port and the target province, each of which has one of your boats in it and no unfriendly boats.

If you're moving troops to your own province, the target province needs a port, and the chain of sea provinces controlled by you must include the one with the target port. (But if you're invading, the enemy province doesn't need a port at all. It's easier to land in an enemy province than it is to land in one of your own!! Although this rule is ridiculous, you'll be glad of it when the time comes to invade Ireland.)

(This is the one that it took me a while to figure out)
When you drag something from one province to another, it doesn't actually move. You've just given it the order to move, but it won't actually be in the new province until you press the "end turn" button. So you can't move the troops until the turn *after* you've set up the boats.