You just have to be able to throw a fairly good score each game and eventually he will have a bad game and you win.
Throwing darts
To throw the dart, click once to pick it up. Line up the point with where you want it to hit (usually the center). Now click a second time but don't let go. An aiming point X is now on the board. Slowly pull back on your mouse, and watch the dart move down below the aiming point. This is "your arm pulling back" with the dart. Pull back until the point of the dart is about 1/2 the total width of the board below your aiming point X. When you release the mouse button, the dart will throw. The lower you went down, the higher the dart will go. So about 1/2 the board width means that your dart ought to hit very close to the X you set. Experiment to find the distance that works the best.
The pipe is a "pennywhistle." You get it when you beat the first darts player the second time, after you have taken the submarine ride and brought your photo back to the pub.
Beat the first player (wearing glasses) and take the rowboat ride, and bring the photo back to Mews. Return to play the other two players to win a submarine ticket (with better darts).
you have to go to the pub and beat the man who doesn't belive in Nessie at darts
You get the pennywhistle after winning your third game of darts at the pub. The first time, you get a rowboat ticket and take the "humps" photo back to Mews. The second time, you win a submarine ticket and get a photo that turns out to be a movie prop. When you beat the original darts player again, he is a poor loser but the bartender gives you the pennywhistle.
The name of the Poptropica island is "Cryptids Island." (see related question)
There are animals on many Poptropica islands. There is a Safari on Nabooti Island, cryptids on Cryptids Island, and in several quests. See the related questions.
There is no Nessy Island. The name of the quest is Cryptids Island. (see related question)
The pipe is a "pennywhistle." You get it when you beat the first darts player the second time, after you have taken the submarine ride and brought your photo back to the pub. Once you have it, take the rowboat across the loch alone (go left twice) and use the pennywhistle in the secluded lagoon.
There are two players you have to beat. The first one, with the mustache, has the rowboat ticket. Once you get the photo on the rowboat, you return to Mews. When you go back to Loch Ness, you play the man who has the submarine ticket. Get the photo and bring it back to the pub. You will now play the first player again.
You only play darts at the Pub to win tickets to the rowboat and submarine rides. The third time you play is against the first player you played. On your first trip to Loch Ness, you get the digital camera and play darts to get a rowboat ticket. You get a photo and take it back to the Mews lab. On your return to Loch Ness, you play darts against a second player to earn a Submarine ride, and photograph the "Nessie" at the bottom of the Loch. Return to the Pub to find it is a movie prop, but beat the first player again and you get a Pennywhistle from the bartender.
You don't get immortality, even on Mythology Island. Collect evidence at the four locations (and later a fifth) to prove the existence of the 4 cryptids on Cryptids Island. (see the related solution) (see the video links below)
All of the darts players are at the pub when you arrive. On your first visit, beat the man wearing glasses to get a rowboat ticket, and take your photo back to Mews. On your next visit to Loch Ness, you will play against the second player, who will give you the submarine ticket. Bring your underwater photo back to the pub and you will get to play the first player again, who proves to be a poor loser.