BY TRAIN
There are four long-distance rail stations - Hauptbahnhof (main station, center), Dammtor (center), Altona (west) and Harburg (south) - Hamburg is perfectly embedded in the high-speed train network of Central and Northern Europe. In addition to Hamburg’s main rail stations, ICE trains call at Lüneburg and Uelzen, and IC/EC services at various other places in the metropolitan region.
With Hamburg as the main rail hub for Northern Germany, all the main cities in Germany can be reached within a few hours (e.g. Munich in 6 h, Stuttgart in 5 h, Cologne in 4 h, Frankfurt in 3 h) and so can the Northern European metropolises of Copenhagen and Stockholm.
Timetable and fares provided by Deutsche Bahn:
https://www.bahn.de/p_en/view/index.shtml
To get to the Red Bull Beach Arena from Hauptbahnhof (main station), just take underground line U1 (direction: Ochsenzoll/Norderstedt Mitte) to Hallerstraße station.
Timetable and fares in and around Hamburg:
http://www.hvv.de/en/index.php