Slightly different issue i had whereby i wanted to adjust the water height independent of the landscape (google suggests this page) although this may also help with this issue.
If you edit the main water material for the lake, ocean, river your using; named something like water_material_[lake, ocean, river] then scroll down to the “Global Static Switch Parameter Val” section and tick both boxes for “UseFixedZ” this will allow you to adjust the water height.
Epic really needs to invest more money in people for documentation/guides/help/support, when doing basic things theres lots of info out there but as soon as you want to do something a bit more complicated or atypical theres a massive void, so many posts or questions where things never get answered.
Hope this helps someone.