In comparison fresh waters seem lonely, barren and free of danger, especially the deep water. There are literally no aggressive animals that can catch you in water.
Freshwater Forageables
Water Chestnut - (shallow)
A simple plant with edible corms and leafs, could be our first wild tuber alternative.

Minnows - (deep or shallow)
Tiny minnows mate and swim in massive groups and often breach water and jump around. You could literally collect them with a hand or bucket if you came across a school of them. Could be fun to see little animated patches of minnows that are harvestable in a bucket, maybe a type of energy food, a bait alternative, a non-fishable type of fish?

Floating Seaweed Pile - (deep or shallow)
Big hunks of floating seaweed like spirulina. Since floating seaweed could be gross and bad, maybe it can be "raided" for a chance to harvest something edible, like the seaweed itself, or maybe some sea snails and mussels?

Freshwater Threats
SMALL BOIS
There are SO many fish in this game, why not add a few that are dangerous? Fishing is practically an AFK snorefest, maybe the addition of a rare nasty fish getting on the line could be fun?
Imagine like, on rare occasion the fishing progress is halted and some splashing animation/sound is played to indicate that you spawned a mean fish. These could also spawn naturally as well and be aggressive or passive-until agrod type animals.
Needlefish?

Freshwater Barracuda?

Piranha?

Stonefish? (BONUS - VENOMOUS!!!!)

THINK OF THE NEW, FUN ENDLESS WOUND POTENTIALS!!!!!!!
BIG BOIS
Okay, so on land we have lots of big fun animals that can fuck us up, in caves, in cave water, in oceans we have the same.
How about a big fresh water fuckdaddy that is actually a legitimate threat? Maybe it's like a mammoth or conch in that it's mostly passive and super rare, but image like a giant super angry turtle motherfucker floating around in a big deep lake!!!

We NEED giant scary turtles tbh. Imagine the fun behaviour mechanics, like having it not go in to shallow or on land, go underwater and move around with some bubble animation and pop up, it could damage the fuck out of boats but be pretty slow and have a fuckton of armour.
MAXIMUM FUN!!!

WOUNDS!!!

EXCITING!!!
please consider