[Gustav Schiebert]

Usually I always attack on the surface for the greater situational awareness - easier to keep tabs on where you are in relation to the convoy and plan a second strike while still executing your first. If you're only looking through the keyhole of your 'scope you can't do this, you usually only get one decent shot. Or if you do get two, it needs some Flank Speed and lots of noise to get in position.

If the weather is clear and the sea (as you describe) it makes use of the 'scope a lot easer. Yes, it's easier to spot for the destroyers but at night, over 1,000m, even a scope 2m out of the water is almost impossible to spot. I'd consider a submerged attack if the destroyer screen was particuarly strong.

Priority 1: sink the enemy
Priority 2: look after your boat (so says Doenitz)

So for me the surface situational awareness would be more important than the safety of being underwater. But that's only for early war.

As soon as radar is in, surface attacks are a no-no. I once tried surfacing in the middle of a convoy in poor weather in the dark, thinking they destroyers would be looking outwards and I could hide on the radar screen amongst the merchants. Despite being over 800m from the nearest ship I was spotted instantly and searchlights lit me up like Blackpool Pier. First burst of gunfire killed the 1WO and two watch crew. Crash dived, located by destroyers and sunk. Not sure whether that was bad luck or radar.