IMHO main island boat 22' or 23' minimum for all around use. Secondary boat in the 15'+ range is fine for nice days and grocery run.
In addition to handling rough conditions, there is the matter of transporting lots of stuff. People. Luggage. Lumber. Groceries. Beer. Hot water heater. Furniture. Beds. Paint. Refrigerator. Ladders. Kayaks. Mattresses. Stove. Tools. More beer.

And so on.
Think of your main island boat as a 4WD pick up truck.