New water flora ideas -
Seaweed!
The world of seaweed can be endlessly expanded beyond Green Kelp and Drift Kelp
I suggest having a baby brother to the "driftkelp island" local resource.
This smaller, randomly spawning pile of floaty seaweed would instead be “raided” like a birds nest.
When harvested you would retrieve any number of interesting water critters, foragables or seaweeds. Sometimes you would get a simple piece or two of some various seaweeds, other times maybe a starfish, fish or mussel, etc.
Seaweed is a crazy diverse edible plant that can have other uses beyond a “salad green” alternative.
Bigboi seaweeds like Badderlocks and Bullkelps are more like meaty vegetable-like alternatives and are often found either washed up on shore, floating around, or growing fresh in both deep and shallow waters.

Some other fun smaller seaweeds like Irish Moss and Dulse are pretty dynamic edibles too. They can be boiled to make gelatin alternatives, eaten fresh, dried and used as a snack or condiment-like additive to flavour dishes.

(they look super similar, but this is Irish Moss)
Some other ideas include shore and ocean plants like Sand Lily's, Sea Lavender, Mangrove Trees or various “sea flowers” like fun and exotic anemones and corals

I imagine these sorts of bright and interesting plants would mostly make for good curios or dye-related crafts.
There are also a variety of edible seashore plants. Beach Peas could be a wild pea alternative, Sea Asparagus could be a simple harvestable snack, Clawberry is a flowering succulent that is pretty much entirely edible and grows fun appendages that look like lobster claws, and the already existing Sandthorn Bush could be repurposed to appear only in sandy oceany biome areas.

(also random fun addition to the sandthorn bush – usually the berries are considered barely edible until they have been bletted, and this could be a fun and weird mechanic that could be added to the game where certain berries and fruits that should undergo bletting actually get increased FEPs if you pick them after they have been left on the tree for a whole winter or boiled. Other examples are medlar and quince.)
Lastly, a couple options for new spice foragables, both Fennel and Sea Parsley grow in sandy rocky coastal areas, could be a fun seasonal, ocean-specific couple of pottable foragables.

Ocean fauna coming soon...