Don - My two cents of a few of your questions:
- Ferries: We usually do not advance purchase unless we are 100% sure of the ferry we want to take. Flight delays can foul you up. Plus, I don't think the ferries are booked up generally speaking and walk-up ticket purchase is pretty safe (except possibly for high season weekend travel). If you have a big group, a private water taxi may be a practical alternative. They typically want you to check your bags (including carry-on roller boards) although its not a problem carrying your purse or day pack.....they don't want bags taking up seating space.
- Boaty balls: We typically don't bother with them, except for Cooper if its crowded. If you are comfortable anchoring you'll have much more flexibility. We have not had any trouble getting FCFS mooring balls the last couple seasons....even arriving late in the day.
-Feb vs April - I don't have a good feel. I've been down in both months and don't recall much difference in the crowds.
See you down there.
Hucker.