FAD: fish aggregating device. There are some.
anchored in deep water under surface. It takes a boat to go there.
GPS location can be recorded to find it. It could be deep enough not
to cause a navigation hazzard.
Going fishing: going fishing to or near vicinity of the FAD. May be
a small surface buy with a GPS coordinate transmitter on it for
Attaching: Fishing boat can be attached to the FAD, and that would
resemble to deep water anchoring.
Strengthening: FAD system could be strengthened to make it better
for deep water anchoring. Multiples of this can be arranged in near
vicinity of each others.
Specially built vessels: building specially designed vessel for deep water
anchoring. I guess, that would be hydrodynamic hulls, could be full of,
or partially filled with, buoyant material, like polyurethane foam, that would keep the vessel afloat and anchored, even if the hull is filled with water.
And so on. After that point specially designed floating structures could be
built, positioned for different reasons (living space, storage, breaking waves … etc)
Regular vessels could be still anchored at such anchoring sites, and used for the same reason (living space, storage, breaking waves … etc).
And so on, and forth…