Sleep aboards are not allowed on the boats at the dock for noise and environmental reasons. Plus the charter starts the next day so the client would have to pay for an additional day. Combine that with the fact that the boat may have just come back from charter the day prior to theirs and it just adds to the issues.
Best to have TMM set you up with a room. They can do all the legwork of finding you a place as not many places will take a 1 night booking in high season.