Hi!
I posted this in another section of the forum already but this might help some of you so here’s a copy 
The water plugin is an experimental version 0.1 and has not been updated for some time. Im sure they are working on improving it. Alright info dump here goes:
The current version is meant for landscape resolutions of 4k or lower.
If you go higher than 4k you will notice once you place down a river the updating of the render target will slow down your editor heavily. Also the Ocean water itself is recommended to be at 0-0-0 world origin, the further your ocean is from world origin the more issues will arise.
The most problem free way currently of using the water system is when the landscape is completely made of landscape splines since they all affect the landscape. So Landscape->Ocean water main island-> seperate islands-> spline mountains on top-> Lakes-> Rivers. This seems like the way they currently use it on fortnite.
Now for the other way’s of achieving some use with this system in its current version.
If you only need the ocean water and dont need lakes or rivers there are a couple of ways of setting this up. I would not suggest importing a heightmap from creation of landscape, because i think the water height is set at middle of landscape height. As a normal heightmap ranges from black to white 0-1, so the water height is around 0.5. and with most heightmaps 0.5 range is already getting into the mountain ranges so your water level would be way too high. You could try to adjust the heightmap in photoshop by setting the levels from its normal 0-1 range but adjusting it to 0.5-1, but this is very map dependant and might not get the waterlevel at the correct height. Either way if this is something you want to try then import the map then set ocean water to additive, select all spline points from ocean water and set all points to location 0-0-0, this will remove the ocean landscape hole or landscape portion.
Other way’s of importing heightmaps, create landscape, add ocean water, then in ocean water settings find displacement. add your heightmap to the texture slot, set tiling to 1, set midpoint to required mid, set color channel to red(1-0-0) then set displacement height to required height of your landscape. This would allow you to sculpt landscape from the water layer itself.
There is also the option of using the landscape gizmo, under copy from landscape tools you can import heightmap to gizmo, copy data to gizmo, set gizmo settings like width height etc then place gizmo where you need it on terrain, then cntrl+v to paste the data onto the landscape.
Now for my personal favourite way currently of using the water system
This is thanks to sjoerd de jong of epic games from the talk “water system deep dive”
It is a hacky method of using the landscape system without it actually affecting the landscape!
Which means you can use heightmaps sculpt and everything. And because it is still using render target it will also cut off the water on the landscape just like before.
In water brush manager you find the 2 top materials: brush angle falloff, and brush width falloff.
Duplicate those, then in the materials on both on the bottom right you connect the Unpack node to the Pack node so that you avoid the landscape changes within the material entirely.
I’l add an image for it in the end of the post.
Now replace the materials on the brush manager with those changed ones and your good to go.
Now do keep it mind that the landscape will not change on its own! So you need to sculpt the river’s and lakes yourself. Hopefully this will help some of you till its out of experimental.
^^
