Hi Rhombus
High season could depend slightly on the weather but May-September is usually the standard.
I completely agree with you - Venice is a must, especially if you haven't been before. I visited Venice for a day trip from Croatia in October once and travelled in a much larger boat. So if the catamaran isn't available (because of the weather or the season), you may still be able to make your own arrangements to get there.
Kind regards,
Olly, your editor