Regarding submerged radio comms.
The Germans had the 'Goliath' system starting from 1943 that could send messages to submerged U-boats. Radio messages could not be sent from submerged boats, however.
The range was extremely long with one U-boat reporting receiving messages while it was in the Caribbean Sea.
Maybe something similar could be modeled in HMS Marulken? Submerged boats at either periscope depth or at the German maximum (I think it was max.) depth of 30 meters could receive radio messages?
Sources:
2.3. UNDERWATER RADIO COMMUNICATIONS IN WW2
http://uboat.net/articles/35.html
5. Use of very low frequency
https://www.ibiblio.org/hyperwar/ETO.../SRH025-8.html