If you miss the 12:00pm private ferry, you can take the pubic ferry that runs out of Casco Bay Lines or call a water taxi. Portland is extremely busy in July, so if you miss the private ferry then you will not be guaranteed passage on the public ferry or water taxi. See the Travel and Accommodations page for more information.
The church is a 0.3 mile walk from the ferry dock - just follow the locals and they will show you where to go! There will be golf carts available for those who need it.
There is no parking for the ceremony or reception. We would suggest leaving your car at your hotel or Airbnb. Street parking is very limited.