In the latest release notes [1.0.3.253] the first listed item is "Coastal Sieges".

How does this work?

You attack a district that’s on the shore with a ship and the siege starts just as it does if you do it with a land unit.

You can also commence the siege with a land unit, but it will start combat embarked, so you will need to make landfall during the battle itself.

Thanks!

I'm gonna try it out.