Yes, there is certainly sufficient room for an 18-ft boat. You can dock on the inside of the float on the north side of the dock, or even under the 2nd deck.
We spent two nights there with a 27-ft boat, and it was a bit big for the dockage, but your 18-ft boat should be fine.
Be sure to check with the management about space for the boat.
Their property includes the water front and they have a solid dock. I'd guess it's at least 50 feet long. The end of the dock has an "L" and an upper deck with chairs. On the L section is a sail boat I think was 30+ feet. There were no other boats on the dock during our stay. So, there is enough room for an 18 footer. There were boats at the neighboring docks too and it appeared to me low tide was no issue. I'm sure if you're staying at the Night Swan they'd let you keep your boat there as long as you arrange it in advance.
Yes there is.
Get an answer from Night Swan Intracoastal Bed and Breakfast staff and past guests.