would the tax deal be the same if you traded in a boat and a vehicle on a boat?
Yes. You would only get credit for the boat portion of the trade. If you look at the TPWD forms, there is trade responses only reflect it being a boat in trade. It's not an oversight. As mentioned, TPWD makes their own rules. They don't want to recognize anything other than another boat reducing the taxes paid. I learned the lesson the hard way.
I traded a motorcycle for a boat with an individual. Sent the paperwork in as an even trade thinking all was well only to learn what I have shared here. Both were titled assets and had to have tax paid when they were purchased. Even though it was an even trade and no money exchanged hands, TPWD said tough, pay the sales tax. Lesson learned.