Go to file/new level/ default then drag across a geometry box across, resize so it is big enough for a swimming pool.
Now copy/paste this, resize to smaller so it fits in the first box and set to subtractive and sit inside first box.
Dead across volume/ physics
In the properties of the physics volume, tick on water volume and physics on contact.
Change fluid friction to 7 to slow it down
Go to content browser then firstPersonBP/ firstPersonCharacter
Now go to top left and click on characterMovement(inherited) then change buoyancy on the right of the screen to 0.5 (to make it bob in the water)
Go back to your level map and drag across a basic/ plane then resize and sit it on top of the pool
Go to content browser the drag across a lake material and put it on the plane on the pool.
In the plane properties change to no collision so the player can fall through the water.