Why build "new" airframes.....why not just modify existing models of aircraft for the maritime role.....say the C-130. The trick to capability is numbers and availability of aircraft.
It has got to be cheaper to mod proven airframes than design-build-test-field new aircraft from scratch.
Subs are fast but even an old Herky Bird can keep up with them and with loads of fuel instead of cargo....could have the endurance to hang around for long periods of time.
Having that big ol' ramp on the rear end from which SAR rescue gear and SF Operations gear could be dropped would be a plus as well. Bring back aerial pickup equipment and it could be a really interesting airplane.