John and Paul
I do hope the project goes ahead, as it would make a fabulous WIP build on the forum!
How about a surface sailing vessel with dynamic diving? That would be a much simpler build without much risk?
Dynamic diving is submerging using front hydroplanes, which could be RC controlled, and the forward speed causes the sub to submerge.
We have lots of subs in the club and their owners very rarely submerge completely, for fear of doing a Thetis!
Bob