The Water Resort is place on water tiles in City Island 2. First you have to buy a piece of the ocean like it was a land tile. The you can place up to 4 Water Resorts for cash. They will build instantly when first purchased with cash, then upgrade fast making them quick to max out at level 10.
For more info visit the City Island 2 Wiki & Guide for help.
The earlier you by the water tile the cheaper it will be. It’s recommended that you buy at least one when Water Resorts become usable when your level is 13+.