Das Boot is both the name of a german submarine movie and also the name of a traditional beer drinking glass. The glass holds 2 liters of beer, or in some cases 2 liters with room for foam (3 liters to the rim). It is the final drinking contest in Oktoberfest and also featured in the movie "beerfest". Literally translated it means "the boat" or in the context of the drinking glass "the boot".
When your girlfriend/wife/boo/mistress/FWB/etc whom isn't a fan of anal sex (greek) gives it up either after you do something exceedingly nice, she is ridiculously drunk, or looking to experiment. Since this doesn't happen very often or takes awhile to work up to, it is like a cruise/trip to those ancient isles in the Mediterranean.
Guy: I can't believe it, my girl agreed to go on a Greek Vacation last night!
Other Guy: Incredible! Was it difficult getting into port?
Guy: You know I am an experienced captain, it took a few tries, but after some maneuvering my vessel just slid right in.