u/tptacek pointed us to the San Remo Manual [1][2].
> could argue there is risk to any vessel, not just submarines
True. The facts and circumstances can vary case to case. But submarines have been de facto excempted from these rules since Laconia.
[1] https://en.wikipedia.org/wiki/San_Remo_Manual
[2] https://news.ycombinator.com/item?id=47560566