Traveling on a budget has helped my wife and I vacation every year to warm weather location.
1) limit your drinking, as drink prices are expensive.
2) do not go to Chops or Portofino as they are extra.
3) excursions can get pricey, there is no need to book an excursion at each stop. Some stops have excellent shopping or site seeing by using a taxi and driving around.
4) at an island when others are on an excursion it is fun to have the boat all to yourselves.
Hope this helps. Enjoy the trip.