First, I don't think your idea of terrorism is correct. By your definition, wars would be terrorism. The word has to contain a notion oft non-combatants or innocents being involved. But I agree with you, that it is pointless to debate semantics especially when lives (of the whales) are at the heart of the discussion.
Regarding your quote from Capt Watson...you take his words slightly out of context with your "..." omissions. For anyone interested, I would recommend going to the link to read the full response which is several paragraphs long. I see nothing wrong with what he said and it shows him to be very well versed in history. It is an accurate account and he is free to have such an opinion.
As for the how they prevent injury question...Over the past 25+ years there have been numerous campaigns, so I'll start with one of their better known ones--the sinking of two Icelandic whaling ships in Reykjavik in 1986. Care in planning is the first step in preventing injury. They chose to act in November, when tourists were gone and the whaling season was over, so the ships would be docked. Prior to even arriving in Iceland, weeks were spent gathering intelligence and devising the plan. Once in Iceland, they spent several weeks conducting surveillance of their target ships until they noticed a pattern. On Friday nights, a lone watchman would be in charge of guarding the four docked ships and he could be seen carrying two bottles of vodka with him each time he arrived for duty. There was no activity on three of the ships, with the watchman staying on the fourth ship which was farthest from the dock. Based on this intel, they chose to act on a weekend and very late into the night when no one would be around. Remember, by now, the crews were ashore, and any work on the ships was restricted to daylight hours, so there was no chance of anyone onboard, except for the lone watchman. Nevertheless, when they boarded the first ship, one person searched the entire ship for any unaccounted sleeping watchman or crew. Meanwhile his partner went into the engine room and opened the ship's salt-water cooling valve which allowed the ocean water to rush in and begin to flood the engine room and ultimately the ship, slowly sinking it to it's rightful watery grave. They repeated the process on a second ship, and by 5am they left the area as half of the Icelandic whaling fleet sank to the ocean bottom. In January 1988, Paul Watson flew to Iceland to challenge the Icelandic government to arrest and prosecute him. They refused to lay charges because a trial would expose their illegal whaling operation to the world.
As for using explosives to sink boats...I do not recall this tactic ever being used by Sea Shepherd. Did you see this somewhere or were you just assuming that is how one scuttles a ship? (On the other hand, the Sea Shepherd has been depth charged by the Norwegian Navy and has been shot at by whalers.) If you are referring to their intentional sinking of their ship, Sea Shepherd, in Lisbon, it was scuttled using the same technique I described above which btw is a slow process that takes place over several hours. (that is what Watson meant when he said "...my crew blew the bottom out of her in Lisbon..." in the linked interview)
Regarding the safety of ramming a ship...Capt. Watson has decades of experience on the sea and by targeting specific areas of a ship at certain speeds and angles, a ship can be safely disabled. Also remember, whaling ships are usually much larger than the Sea Shepherd ships, so it is not a high speed car wreck like you may be imagining. Plus, ramming is used to disable a ship, not sink it.
When there is danger or the chance of injuring crew below deck, they have refrained from engaging and often the whalers get away and must be located another day. Also, I do not think ramming is their tactic of choice anyway.
Well, hope this clears up a few things for you. It's easy to imagine dramatic dangerous situations when in real life they are not. By the way, you may also be interested to know that Paul Watson was named by Time Magazine in 2000 to be one of the world's top "Environmental Heroes of the 20th Century." And in 1999, he was awarded the George H. Bush "Daily Points of Light Award" for his conservation activism.