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?

Why do the drones need to be able to work underwater? Can’t you just connect some floats to them and launch the drones once they’re at the surface?