Not being familiar with the San Remo manual, a quick review says it does not contradict the 2nd Geneva Convention but it does not seem to directly address shipwrecked survivors. My read of GCII Article 18 [2] seems to clearly make this a requirement however.
My focus was on the the inhumanity of torpedoing the ship given the situation. Are you implying you disagree with this?
[1] https://www.washingtonpost.com/national-security/2025/11/28/...
[2] https://ihl-databases.icrc.org/en/ihl-treaties/gcii-1949/art...