D&D General Things to Find in a Large River?

[Tallifer]

Our dungeon master placed a deep pool just downstream from an enticing ford. Four water nymphs emerged and charmed our half-orc into diving into the water toward them. However it turned out to be the illusions and mental trickery of an aboleth in need of new thralls.

 

[Azzy]

Scrags, lacedons, bullywugs

 

[Zardnaar]

A magic ring that makes you invisible when worn, lost long ago by an ancient evil sphinx-lich, commonly assumed to be long dead.

I like this one, might make it a different type of undead.

 

[Zardnaar]

Dammit gonna copy and paste a few of these ideas as plot hooks and red herrings.

How about a Dragon Mummy?

 

[Leatherhead]

Not unless you consider lifeguards "regular people". Gives a whole new perspective to the Baywatch series.

Long live Reverse-vampire Hasselhoff!

When he bites you, does he inject extra blood into you?

 

[Lidgar]

When he bites you, does he inject extra blood into you?

No, just chest hair and the ability to communicate with Pontiac Firebirds.

 

[jasper]

Water.

Heavy water, Light water, dry water, Black water.
Swords. Bodies. Ruins of London Bridge.

 

[Quickleaf]

Funeral barge for a wealthy priest with dancers, trumpeters, and an orator-priest speaking of the deceased's good deeds. It is preceded by a procession of giant swans (swanmays) wearing golden headdresses and "dredging" reed canoes go ahead of the procession, manned by fishermen equipped with long poles to dredge up any dead bodies – as in this culture sailing/boating a recently dead body over other dead is considered to tempt the evil eye. Dead bodies are pushed to the side of the river to be hauled out by porters being paid in silver and turned over to the local Mete-f (Pharaoh's police) for investigation into cause of death.

 

[Zardnaar]

Bodies in Per Bastet get animated

 

[Hussar]

A good source of inspiration for you:

Great Library of Greyhawk

greyhawkonline.com

The Great Flanaess River Adventure. It's not specifically 5e, but, the ideas should work.