The pennywhistle is said to summon the creature.
After you have made the trips in the rowboat and submarine, you get the pennywhistle from the bartender (the dart player is a poor loser). Go out in the rowboat alone, go left twice to the lagoon, and use the pennywhistle.
The prize for seeing Nessie on the Cryptids Island Preview was a Nessie hat.
The prize for spotting Nessie during the Cryptids Island preview (beginning December 2, 2010) was a Nessie Hat. (see related link)
Dive to the bottom and then go left.
During the special preview of Cryptids Island (beginning December 2, 2010), anyone who spotted Nessie got a special Nessie hat. However, you could not always see Nessie, and when you did she was a distant shape on the loch. (see related link)
Yes, you can find proof of The Jersey Devil, Bigfoot, Nessie, and the Chupacabra. (see related question)
If you use the sub, I think you will find/see? Nessie.
The viewfinder game was part of the demo of Cryptids island and had almost nothing to do with the quest. It was possible to see a hazy outline of Nessie after you saw the dog barking. If you did, you received a Nessie costume prize.
There does not seem to be a horn on Cryptids Island. There is a flute (a pennywhistle) which you receive after the submarine ride, after you defeat the sore loser at darts again.
In the same part where you find Nessie. It is in a pub at Loch Ness. Go right past the ticket stand and bridge.
You need a photo of the actual Nessie, a tooth from the chupacabra, and egg shells from the Jersey Devil. (see the related questions below)
Along the bottom, you will find a brightly-colored Nessie face and get a photo of it. Bring it back to the pub where it is revealed to be a movie prop.
There was no way to see the actual monster on the Cryptids Island preview. Likewise, there is no way to see Nessie with the telescope in the actual quest. All you see is items disappearing into the water. You finally get to see the real Nessie on your second visit to Loch Ness. Having gotten a photo of some "humps" in the water, you find that they are just truck tires. But the real Nessie is a shadow in the background. After another attempt aboard the submarine, you eventually lure the creature to the surface with a pennywhistle (flute) and get its photo. (part of two long visits to Scotland in the game - see the related question)