I would like to set up a swimmable ocean , but a physics volumn is not working correctly here , because the volumn is rectangular in shape, and I want the player to get affected by it only when in touches the ocean and moves a certain distance. A rectangular physics volumn not working acurately here and at some part of the beach ,land area may overlap the physics area.
The default volumes are set to be a square. You can use the details panel > Brush Shape to select a volume style that best fits what you need. You may need to just use the Cylinder and then go into Edit mode so you can adjust the vertices to better fit your needs for your volume along the shore line for your water.