Just how quiet are these fan-jets as far as putting noise into the water. AKAIK the Nimrod was much quieter than the P3 but either could be counter-detected on a sub-sonar array.
RR has invested a lot in new rotor technology and says Stage 4 is within reach. The supersonic shockwaves from the blade tips are reduced.
I can imagine that optimizing for low noise goes at the cost of efficiency. I guess it could be introduced as an options on engines if required. A "silent" mode to be selcted by the crew. Different rpm's, blade angles..
