The objective in Find the Treasure! is, as the name indicates, simply to find a treasure. To formulate a strategy for this, we need to establish what the treasury is and how it's best found.
The treasure as such doesn't exist on the game board but is found by drawing the right pair of the three cards along the horizontal edge of the game board and the three cards along the vertical edge of the game board. The cards come in three kinds: two treasure (+1 each), two empty (0) and two curse (-2 each) and you need +2 to be considered having found the treasure. Since you draw two cards every time you dig, you need any of the following possible combinations:
1 treasure + 1 treasure
1 treasure + 1 empty and 1 treasure + 1 empty
1 treasure + 1 treasure and 1 treasure + 1 empty and 1 treasure + 1 curse
1 treasure + 1 treasure and 1 treasure + 1 empty and 1 treasure + 1 empty and 1 empty + 1 curse
Since you can't draw more than 4 treasure cards without also drawing a curse card, you can't win if you end up with more than 2 curse cards. The first combination in the list only requires 1 digging, the second 2 diggings and so on. Obviously, the sooner you find the treasure cards, the better.
With the treasure identified, let's see how it's best found. Each player may preview 1 card when the game starts and the card you preview determines your strategy. Generally, you should narrow your search to 2 potential treasure spots to have a chance to dig at both and escape with the treasury. This means that either one of the spots has 2 treasure cards or none of the spots have curse cards.
Treasure card previewed (the lurking game)
If you're lucky to preview a treasure card, you have limited your search to 3 spots. By digging at all 3 spots, you will find 3 treasure cards and only 1 curse card - enough to win. However, you're not alone on the island so if you start digging immediately and your opponents are close, they will most likely attack you before you manage to escape with the treasure. If they are not close, you may gamble and start digging, hoping to find enough treasure cards in one or two diggings. If they are close, it's better to pretend that you didn't preview a treasure card and instead moving towards another player.
If you see someone else digging, you can deduce that the spot where your card and that player's card intersect is the best place to dig and go for that. If not, you can either try to lose the closest player or attack a player to see her card and narrow your search before you start digging. In the latter case, you should of course attack a player with a card from another edge (i.e. horizontal if you previewed a vertical card and vice versa) since you already know which card on your edge that is a treasure card. The key thing to remember is that you have a safe escape route back to your ship once you find the treasure.
Curse card previewed (the attacking game)
To preview a cursed card is worse than previewing a treasure card but still better than previewing an empty card. You will now know which card to avoid and have limited your search to 6 spots. You will still have to attack another player to see her card and narrow your search and the best player to attack is a player with a card from another edge, not because it gives you the least potential treasure spots (both options give an average of 3,33 potential spots) but because that's the option that may narrow your search down to 2 spots (if the player's card is a treasure card). If that happens, dig and escape as soon as you can, otherwise you must either attack another player or play the bluffing game (see below).
Empty card previewed (the bluffing game)
To preview an empty card is the worst start since it doesn't help you at all. Even if you attack a player who turns out to have previewed a treasure card, you still haven't narrowed down your search to 2 spots. On the other hand, your card will help them so you run a risk of making your situation even worse by attacking. What to do then? The anwer is: dig!
What? You don't even know where to dig. No, but the other players don't know that you don't know. By digging randomly, you will confuse the other players. If your action triggers another player to dig, it's likely that that player has previewed a treasury cards and then you know who to attack. If your action triggers another player to attack you, you will either get to preview her card (if you win) or get rid of the bad cards you got from digging (if you lose). And if you don't want to lose your cards because you happened to find a treasure card? Well, then you've been lucky after all and should only try to escape. How? Well, the escape game is the next topic.
About the terrain
If you need to escape from the other players, either because you want to keep your cards hidden from them or because you want to escape with the treasure, you can use the terrain at your advantage. A swamp is an excellent way to lose your followers, since they can only be crossed if a raft is on the right shore. A cave is another, since they can only be exited through a random exit. If a bridge is on your way, try to get on top of it so that you can attack pre-emptively with an advantage. If things come to worse and you get defeated and forced to return to your ship, find a shore to sail to before you leave the ship that enables you not only to strike back but also to escape again before the other players get close.
In conclusion, no matter which card you preview at start there are strategies that will help you. Lurk if you preview a treasure card, attack if you preview an empty card and bluff if you preview a curse card. Then take advantage of the terrain to escape. Fair play is not a winning strategy to Find (and keep) the Treasure!