You can use a lower res texture but you will have to do some changes so that you can pass the original landscape size manually via parameter… currently if you plug a low res texture then it will assume your landscape is smaller and therefore will not match the actual landscape in the game.
Open BP_Ocean and check out the Set Global Parameters function, the heightmap size x,y is taken directly from the texture via GetSizeX, GetSizeY… you can switch that to a variable which then you can manually set.
You will also have to do a similar change in the OceanManager.cpp thought so that buoyancy is synced 