I think this riddle has garnered as much attention as it is going to, so I figured I'd fill some of the gaps "for the record" with a stanza-by-stanza answer.
Where am I leaving?
The Baltic Sea
No more stallin', no excuses,
Berthel's ink is almost dry.
I've seen wars--awe and terror,
Contains Tallinn, Helsinki, and Warsaw--all capitals of countries which border the Baltic Sea. Berthel, according to Ancestry.com, is a Swedish/German name, two more countries which border the Baltic Sea. "Stallin'" is additionally a reference to Josef Stalin, the leader of another nation with Baltic Sea shoreline (USSR/Russia).
Now it's time to say goodbye.
Goodbye, cool waters!
Adieu, rocky shores!
These are just literal and were meant to point you in the direction of a body of water.
I'll leave you with the friends you've made.
Obscure and a bit morose, but this refers to the Baltic Sea being a mass grave due to naval warfare during World War 2 (couple this with Stalin and "I've seen wars" for a deliberate WWII theme). Typified by the largest maritime disaster in history (by loss of life) with the sinking of the Wilhelm Gustloff.
I'm off to brave
The narrow straits
In search of a world in which to trade.
Refers to navigating the Danish Straits to reach open ocean, and thus "the world".