Any idea what the case capacity of water will be? It sounds like it would be just a few grains larger than the .308, so standard .308 loads would be a good place to start.
Alternatively, you could go play with the trial version of Load From a Disk at
www.loadammo.com and see if you can input the data to get loads.
Lacking that, if you supply some basic data, I'll run it though my own copy of LFD. I'd need case water capacity, barrel length muzzle to breech. case length, bullet length and weight, and an approximate seating depth.