I like your thinking GoldenRivet. I figure that a Passenger/Cargo may have some special war-winning weapon or research hidden in its hold and is therefore fair game. A fishing boat? Fairly unlikely it's god anything more valuable than cod onboard.
As it was, it didn't even know I was there as I submerged only 3:15 (for a speed measurement!) after the lookouts first spotted it 11km off. I let it on it's way. In future though if one detects me, zig-zags and I'm surfaced, I'll probably rake it with 20mm.
Chivalry lives on!
p.s. Life rafts??? Where do you spot these?