ok, assuming you have your water on just a large plane that intersects with the landscape and not actual meshes built to the right shape. fastest way and least effort i can think of is to take a top down screenshot of the water, take that into 3d program and use as an image plain. use it as a guide to build a proper mesh. make it as simple as possible and make sure it is higher than the player. import to ue4, place over the top of lakes, set to not visible in game. now open the mesh and set use complex as simple.
this works great for simple stuff that has awkward shapes without having to create custom collision
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