I am currently working on a yacht.
I cannot build outside of the boat template, although I’ve seen EA premade homes with little dock buildings! I am yet to figure out how to do that! My current boat currently has a basement for my 5-Sim family bedrooms with 2 bathrooms. The first floor has a kitchen, living room, and a dining area, while second floor is constructed out of the study/control station and leads to the roof with a hot tub! My Sims have 18,000 simoleons left over (from a total of 61k)!
Once at the Edit Town, you can choose from premade houseboats, or choose a residential “dock” lot, and from a drop down menu select the type of houseboat template you want to use. There are currently 4 options total, which include 2 different houseboat template types.
Here is a photo of my houseboat in progress! More on Island Paradise soon!