I want to block characters so they cant step on / walk over the water. Marking dont allow to step on, does not work.
Alternative I can make a lot of blocking volumes, but the landscape/lakes/oceans are not nice squares, so it would take a lot of time to set up.
Is there any other way?
Is it ok to bump after some time…?
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
welcome to game dev 101 
In case anyone else comes looking here: Add a navigation modifier component to your water BP. The default settings should work. Make sure the area class is set to Null. Then rebuild navigation. By pressing “P” in the editor, you should be able to view navigation-able areas. The water covered areas should no longer appear green, nor should they be red. They will null all navigation in the overlapped area, as far as I know. Hope this helps someone.
i realize this is old post, but people will still look it up for advice. There is much easier way to create collision volume from a water plane than manually placing cubes, and it scales no matter how large the terrain is.
Export your terrain and the water plane as an fbx. Bring it into your max, maya, blender, etc.
Boolean the terrain and water plane together. Now you can delete the terrain portion of the mesh. Extrude the edges of the water plane up and down. That is your new collider mesh you can send back to unreal. It will match your terrain perfectly, no matter the scale. Ezpz.
It will take about ten minutes to do this once you’ve done it once already.
