Think about it like a box surrounding the water surface Cygnus, the water is reflecting whatever it's angle (or it's vertex normals if you like) is pointing at, in most water systems you are viewing the water surface from above you will never reflect what is underneath.
An underneath reflection could only happen if you made the waves so violent they crested over like in a surfing simulation, which is quite a hard effect to achieve in realtime. However, if you made your waves like that you will actually reflect the water below and not the seabed, which wouldn't work with a cubemap anyway.
Therefore I personally cannot see a need to render a down surface when doing a reflection cube map for a planar surface like the ocean.
An underneath reflection could only happen if you made the waves so violent they crested over like in a surfing simulation, which is quite a hard effect to achieve in realtime. However, if you made your waves like that you will actually reflect the water below and not the seabed, which wouldn't work with a cubemap anyway.
Therefore I personally cannot see a need to render a down surface when doing a reflection cube map for a planar surface like the ocean.