> I'm almost surprised this doesn't already exist after looking at it.

What's the use case? We have very good dedicated aerial drones and very good dedicated submarine drones. If something is technically possible but no one makes it usually it means it's not that interesting

If your mothership is low-observable/submersible you could park it in a harbor and when time is right unleash a swarm of hybrid aerial underwater drones with mission payload.

This design uses 1 swashplate server plus 1 brushless drive motor for each prop for a total of 8 motors. Designs in Ukraine are now using 1 brushless drive motor to synchronously drive all 4 props and 1 swashplate servo each prop, reducing total number of motors to 5, which might be a natural path of optimization for this type of hybrid aerial underwater?