I used two of Seatrans ferries a couple of weeks back and they were, quite frankly, scary. As i lived on Samui for some time I use both the mainland vehicle ferries many times and with a lot of trepidation. I've spent a lot of time on ferries around the world notably the Spirit of Free Enterprise - you then spent a lot of time looking to see what makes a boat safe in the event of a mishap and the Seatrans I went on did not inspire confidence.
The design of some of the boats and the vehicle decks would not be acceptable in many countries the life jackets were not properly accessible I've ever seen thm moving with the loading doors open.
I don't know how often these boats are inspected, but I do know how insectors are "treated".