Thanks for the reply Theo! No, it wasn't a fresh project and indeed that was the reason the error occurred, it was a silly mistake, but I'm posting the solution here so if anyone encounters this same issue can use this as a reference.
There was another event running on parallel on the map that was turning the transparency off, so the easy solution was to erase that event and that was it, but, in my case, I needed that event, so I simply modified the event with a condition, evaluating if the actor was inside the boat, and if he was, I set that the transparency mode should be on, and if not, it should be off.
Thanks again!!!!