Also see seagrass fields

Lots of aquaculture-seaweeds (nori), shellfish, seagrass (fiber),

Stellers Sea Cow-look it up-

Also - go deeper than 20 ft-say most is under 20 meters but with canyons and such deeper. And even those regions could have kelp forests.

Barges would be surprising common. Actually the tech of the ships may well be pretty low-seaworthyness not being a huge challenge.

Look at places like Holland, Haiphong Harbour boat people, Swamp Arabs, and Venice for ideas on how people live on shallow water.