I really don't know. Presumably from an inflatable in some way or other.
- which is interesting in that it suggests they were already attacking and interfering with the Yushin Maru. Paul Watson says they boarded the ship to deliver a letter - which makes little sense if their other actions happened first.
They boarded from an inflatable. The only thing that the boarders had with them was the letter.
Sea Shepherds boarding
Here is the letter they delivered:
Sea Shepherd Letter