We have made ferry reservations every time. Marjorie with Island Magic has helped us. magicplace@hotmail.com
It seems that it is not a specific time and boat per say, but if you miss a connection they will get you on the next available ferry. Usually we arrive Delta about 120. We have never made the 230 ferry, but there is one that leaves at 3. On the way back its usually the 11am. I'm sure there are bad experiences, but we have had good luck with this one. Enjoy