Hmmm...
From what I have seen of the actions of the Sea Shepard Conservation Society, it is most likely that the Sea Shepard came alongside the Yushin Maru 2, from there, members of the crew threw the bottles of 'rancid butter' over at the decks, possibly injuring a couple of members of the japanese crew in the process. As for how they got onboard, that is quite simple, if the decks were cleared because of the hurling of large bottles of 'rancid butter', it would be very easy to run an inflatable dingy up the slipway (have you ever looked at the rear of a whaling factory ship?). The usual method employed to prevent this is the use of high pressure hoses to wash the invaders off. If the deck was littered with 'rancid butter' could this have been done?