No, a lorry loaded with explosives would not work because you cannot take your vehicle onto a ferry without proving you are the registered owner or that you have ID and authorisation to drive it abroad. One carload of young people who turned up in their employer's car to board the vessel were denied entry because the driver hadn't realised he needed to get the paperwork from his employer to gain entry on board. Yet having said that, there was one container lorry that was found to have gained entry to the ferry yet it had no registered driver.
An explosive device OTOH is cheap and effective. Semtex is like plasticene in the hand and - like many plastics - only becomes dangerously explosive if it is wired to detonate. So yes, if you were a military agent wanting to send a message to the west to stop smuggling out Russian state secrets, then you use military means which are strategically placed devices, or similar. Your aim is to stop the ship - and your state secrets - from reaching their destination and it true military style you don't just make one attack, you have a follow up one to make darn sure. Just as the German ship carrying civilians and military personnel in Operation Hannibal in 1945 was struck three times by the Soviets, once in the bow, once in the engine room and once in the auxillary quarters so a precise military operation would be used by the elite speznats.
"The first was nicknamed "for the Motherland", the second "for Leningrad", the third "for the Soviet people", and the fourth, which got jammed in the torpedo tubes and had to be dismantled, "for Stalin"."
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So saying that it could have been done another way is just silly. The Russians were sending a message to Sweden and Estonia and they
wanted them to get the message.
Think about ti, a series of three explosions in rapid succession went off at Swedish midnight in international waters and within 48 minutes it was at the bottom of the seabed.
ibid