In my opinion it was. I always ask myself who benefits more if we don't know the truth and for me it's clearly Russia.
The bridge was still usable after the attack. One lane got destroyed and the train track got damaged. The cars used just the other lane for the time being and the train tracks were repaired pretty quick if I remember correctly. Further, they scan and inspect every truck driving on that bridge.
That leaves a boat? Could be, but pretty unlikely in my opinion, because it had to go a long way around crimea unnoticed and because only one lane collapsed and the other had no damage at all, which makes a truck bomb much more likely. And who benefited most from that attack? Russia of course, because soon after they started their missile terror right before the winter as an answer to that bridge attack. A pretty good trade for a half usable bridge for a few months in my opinion. I think that incident was needed by Putin to justify his civilian terror. He chose a prestige object worth mentioning, but did not hit it that hard for it to become a real problem.