Let's have a moment of silence

[GoldenRivet]

rammed by a merchant i would guess.

were you IN the convoy or way off to their flanks?

i have been grazed by a small merchant once or twice, but a convoy moving at medium speed - and if your hit by something with a deep enough draft say like a tanker or a large cargo, it would likely be enough to inure a man and cause flooding to the boat.

another possibility is that your periscope was spotted and an armed merchant or an escort fired at the periscope striking your sub in the process.