Thank you!
I am still working on the submarine, however it is mainly done. After doing 3-4 test fights with sub to sub combat, I found that conventional weapons work well underwater, and a good hit will cause the sub to go for a dive straight to Davy Jones locker.
Sadly, I could not bring it to another save and have it AI controlled, the AI pilot tends to surface, negative the entire advantage of it being able to dive.