Offshore Construction
To build a base on water you will need some floating foundations. You can buy these from a Vending Machine found at the Large Fishing Village.
These foundations come in two shapes; square and triangle. If you have ever built a base before you can probably figure out how they work and how they fit together. The square is priced at 25 Scrap/each while the triangle cost 15 Scrap/each.
A floating base
Find a location you wish to build on and put down your first floating foundation. From there you build “roofs” (the regular type). This will consume floating foundations from your inventory while you are expanding on the surface. Also remember to put down a TC.
Netting and sharks
If you build wall frames under your floating base and place some netting in them, they will collect random junk, ready for divers to collect. Mind that collecting loot this way may attract hungry sharks.