Houseboats

 are seen in the Mysterious Island, in the water. You unlock them by leveling up the island Monuments to Level 5. Each monument upon reaching level 5 gives you one Houseboat slot.

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.

The Royal Wavecutter
This is a really beautiful looking houseboat that costs 100SPs. It's a two story, two bedroom house with a double bed in each so two couples could fit in here easily or one of them could be a child's room. Downstairs there are two really good sized rooms, the kitchen and the living room, the dining table is right in the corner of the kitchen. Glass doors lead out to the outside area which has a small swimming pool in it. However, it does mean there is no room for gardening plots. There is a nice size bathroom upstairs but no bath although that could be easily added.