Whenever possible buy tools, the Machete, Pickaxe, Dynamite and Bug Spray items can be found in the 'Tools' section of the Store. Usually you will be prompted to purchase each tool as you need them but having them on hand boosts the chance that you'll find fruit or additional Gold during the course of treasure hunting.
Don't stop playing if you end up finding ALL of the treasures on each island before actually searching each grid as you'll get a major Experience/Gold bonus for completing the island itself. Plus those last unsearched spots may have hidden fruits and other gold stashes.
When you first begin the game your home island will feature two small plots of farmable land and you will have a choice of four different fruit seeds to plant. To get the best return on your time plant Watermelons.