Aside from regular buildings and decorations on land, there are also some unique structures in your town that can only be placed on water.
Check out the tab marked with a water drop icon in the Store to see them. The decorations, community buildings, and factories that you'll find there can only be placed on your town's bodies of water within the red buoy line that marks the limit of your construction zone.
How do I place buildings and decorations on water?
Last Updated: 6h