How to make player go from walking to swimming within one scene?

dylanbell

I'm making a platformer and I'm wanting one of my levels to be half land and half water. I want the player to start the level on land but at some point have to jump in water and swim the rest of the way. How would I get the physics for the player to change from just pressing on screen buttons to walk to pressing on screen buttons to swim? I want the player to press on screen arrow buttons to choose the direction and i want them to also press another button that takes them further up to the top of the water and if they dont press that button I want them to sink to the bottom. Thanks!