> They could have surfaced (remember, it was KNOWN that there was no threat)

It was by no means "KNOWN" that there was no threat. A modern submarine is inherently in a much more unsafe posture when surfaced, which is precisely why they never do that, especially when it's possible to encounter an enemy.

> tossed out a few inflatable life rafts.

Why do you think submarines randomly carry inflatable life rafts? If they had enough space for those they'd toss them overboard and load additional food stores instead.

Moreover, a surfaced submarine close enough to a floating group of survivors is actually dangerous to those survivors. It has a rotating screw at the back which can seriously injure or kill people and it's not like there's a deck trebuchet equipped to lob life rafts at a distance, even if it carried them.