Well, I've started to mix and match different shaders in between other work on my World War I mod. It looks like you can get SH5 shaders to work.
I have gotten the SH4 water to imitate some aspects of SH5 water. However, it will take a lot of trial and error to make a high quality environmental mod. So far, the end result of adding SH5 water to SH4 does not look as good as the best SH4 environmental mods.
My main problems right now are:
1) how to get a more rippled effect to the surface water
2) how to get the little flecks of surface light to reflect from a greater area on the water
3) Not related to shaders, but how to get sea foam at a lower sea state