Ok thanks. I’ll try that.
For those who don’t want to play forum hop scotch… his answer in the other post is
" Yep, I finally found the answer. For anyone who is looking to expand the underwater post processing size here is the answer New 4.26 water questions - #16 by kernelkiller . Select your WaterBodyOcean instance go to Collision and change the X,Y of Collision Extents. Now you can have an infinite ocean. Also to increase surface size. Select your WaterMeshactor > under heading Mesh > increase decrease x,y of Extent in Tiles. By increasing Extent in Tiles and Collision Extents you can make an infinite ocean."