This is a paid asset, but now you can download Third Person Swimming Add-on Free.
Detail this asset from Unity Store: Original Link
Invector – Third Person Swimming Add-on v1.4 (Latest version)
“If possible, please buy the package to support the developer”
* THIS ADD-ON REQUIRED THE BASIC LOCOMOTION PRO 2.5.0 TEMPLATE *
Swimming Official Add-on Plug & Play
This swimming system allows you to swim above or underwater, you can trigger particles effects using Events that knows when you’re on the water.
With this add-on, you not only get the swimming but also the knowledge to create custom modular add-ons without changing the core of the controller.
* Required Basic Locomotion 2.5.0 or higher, if you need a previous version, please contact support ** MeleeCombat & Shooter already contain the Basic Locomotion