Boathouses are seen in the Mysterious Island, in the water. You unlock them by leveling up certain island monuments to Level 5. For example, when you level the Riches Of Terra to Level 5, you unlock a house boat.
Types of Boathouses
The Tiny Tight-Pocket
The Tiny Tight-Pocket is the cheapest houseboat but it still costs 20SPs. It's a one story, one bedroom houseboat that could fit 2 Sims.
The Cavernous Cruiser
The Cavernous Cruiser is a two story houseboat with room for 3 Sims to live here, and it costs 40SPs. This boathouse comes with one bedroom, a good sized living area, a kitchen, and a bathroom. It also has a garden plot outside the house and a good sized garden area.
The Pint-Sized Prince
This is a one story houseboat that costs 65SPs. This is a good sized houseboat for a family of 4 although they have substituted the garden area for those extra and larger rooms which is a shame. This boathouse is bigger than the others, making it cost more.