Cool suprises like that are great!
Snuck into a Convoy in AK the other night that seemed oddly spaced. Find out later it has two burning ships from an attack (and a Liner which made for some nice tonnage
).
A limping escort trailing way behind. Out of eels I waited next to a stopped but unsunk tanker and when the gimp got close enough I surfaced and finished her off, and the tanker. Almost got nailed but it was cool.