The first thing to do is Save Early, Save Often.
After you sink the tanker, SAVE.
After you're out of the Kiel Canal, SAVE.

Basically save once you've done anything you don't wanna do again.
Also, don't use super high TC when there might be ships around. Middle of North Sea? It's Ok to try 3000x TC. English Channel? Don't go above 512x TC. Exiting Kiel Canal, try to stay below 128x TC. Open Atlantic? 8192x if you can handle it, just remember you can still hit things! heh
I basically never go higher than 512, except for the long open ocean trips at 2048x maybe. But again, save before you do that.