We had the same plan 2 years ago. No problem with the ferry and taxis or the check in at the Mariner Inn at about 6 pm, but there was no chart briefing that evening. The next morning, being a Saturday, was bedlam at the Moorings docks. We ended up leaving the dock at about 2 pm. You may have a leg up, since you will already be on board, but don't count on it. Best advice I can give is to relax, you're on vacation.