What are the Federal Zone and a beach concession?
The depth is measured from the Federal Zone, which is set back 20m or 66 feet from the high water mark and you cannot build a solid live in structure within it. The Federal Zone is not owned by you but may be enjoyed by you. Therefore, it must remain beach but can have palapas/cabanas, a pier/dock, etc. You simply cannot reside in it. To build a dock or gazebo in the Federal Zone you need to have a beach concession permit, which is very easy to obtain.