Suggestions - probably been suggestified already...

[Prometheus]

Diving board & real dives to learn
Harpoon gun/grappling hook for hunting/climbing
Simple raft/canoe

[Animosity]

The water area is not big enough to need any of this lol

You have diving gear for a reason

[FoamySlobbers]

Harpoon is called a trident. throw it under water for instant death.

[weirdee]

They used to have water striders and water skating, but there was no real purpose for this and they removed them. In the same vein, all a boat would be good for is finding out that trying to fight mosquitoes on a destructible structure is a bad idea.

For the most part, max swim speed and a bubble helmet will pretty much handle everything with no issue.

[Prometheus]

These suggested items are just for fun, not necessarily as tools. Wants, not needs.
Though a crossbow mod to fire harpoon/grappling hooks on a tether you can climb or swing from would be useful.

Why not throw in more recreational items to play with?

[contastic6418]

They actually used to have a harpoon gun in the game but removed it because it didn't work how they wanted it to. There's more info on the grounded wiki page.

[Prometheus]

@contastic6418 Thanks!