When the tanker Sirius Star was hijacked some in the media expressed surprise that a supertanker could be boarded by pirates using open boats. This picture shows however the height to be scaled is not all that far. It looks difficult but not impossible. A ladder equipped hooks at the top may have been used. There are likely a good number of people in Somalia who would be willing to give it a try for a share of three million dollars.
UPDATE: Sekulich , in comments points out that this photo is somewhat misleading, I agree. I intended to illustrate that the freeboard of a loaded tanker is less then what people may think. Not shown is, for one, the powerful wash that comes off the prop of a ship at full sea speed. It is much more difficult to board a tanker at full speed in the open sea then at anchor on a calm day which is what the photo shows