I didn't even notice that some of the attractions close at 7 - thanks for letting me know!
How long do the boats take, is it fairly quick to get from 1 side to the other?
In my experience, they are reasonably quick to get across and certainly way quicker than walking once you are on the boat, however we never take them unless we either see them pulling up or they are there already. This is because unless you are going literally 180 degrees, when you add in the sometimes significant wait they are not quicker than walking. That said, I believe rlovew was suggesting to use them if you get partway around and DD is not enjoying all the walking, so in that case they would definitely be the way to go as time is not the main consideration.