Bremerhaven has also a regional airport and you can check the website for further information:
There's also a ferry connection between Nordenham-Blexen and Bremerhaven:
by train (ICE + RE) from Berlin Zoologischer Garten to Bremerhaven - ca. 4h
for connections check the timetable here:
You can get around in Bremerhaven quite well by bus.
Timetable, ticket prices and plan of the bus network: