I did some looking and this is a non-trivial problem with no simple model that is able to predict outcomes. Most models are computationally derived and are difficult to make into a game mechanic.
That being said, I did find an article in the National Medical Library which is part of the National Institute on Health which looked at outcomes from various recorded underwater explosions. While the entire article looks at many different aspects of this, including evaluating protective measures advocated over the years, there was one chart that I think can be the basis of some underwater explosion rules. The article can be found
here.
Figures 3 through 6 nicely combine and simplify the dataset used and can probably be the genesis of a convenient rule for DWP underwater. If you want the actual data set, you can download it as well from a link towards the end of the article. I will try and build a simplified rule set for underwater explosions in the next few days if my schedule stays relatively open.