You don't. When you are in the submarine and find the Nessie "head", position the light on it and take a photo. Show it to the dart players at the pub.
You win at darts a second time and get a submarine ticket.
No. You only go down in the sub once, then bring your photo back to the pub, where it proves to be a movie prop.
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.
When you get the submarine ticket at Loch Ness, dive to the bottom and then move forward. You should see a Nessie "head" sticking up. (If you look past it, you see it is a prop.) Take a photo and go back to the pub.
On the submarine tour, dive to the bottom and you can photograph a colorful "Nessie" head. But when you take it back to the Pub, it turns out to be a picture of a movie prop.
Bring the submarine photo back to the pub, where a newspaper clipping shows it is a movie prop. But the first darts player returns to challenge you. Once you have the pennywhistle, use the rowboat to go far left in the loch, to a secluded cove where you can call Nessie.
You get the submarine ticket from the other two players in the pub, by winning at darts. Take the picture from the submarine back to the pub, where you find it is part of a movie prop. But the first dart player angrily returns and wants a rematch. Beat him to get the pennywhistle (from the bartender, not the sore loser).
The second picture is taken from the Submarine, and you bring it back to the pub. But it shows a movie prop. Beat the first player at darts again, and you get a Pennywhistle from the bartender. Take the rowboat out alone (go left twice) and use the pennywhistle to lure Nessie for a picture.
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 proof is at each of the locations, but there is some less-than-convincing evidence. You get a photo of Nessie at Loch Ness that is faked, and another one that is a movie prop. But you finally get a real photo. You find some fur on your first visit to Puerto Rico that turns out to be coyote fur, but you return to trap the chupacabra. You find a footprint in the snow in the Himalayas that is unconvincing, but when the three other cryptids are proven, you get to hunt for Bigfoot instead.
On the submarine tour, dive to the bottom and you can photograph a colorful "Nessie" head. But when you take it back to the Pub, it turns out to be a picture of a movie prop. Beat the first dart player again to get a pennywhistle from the bartender. Take the rowboat out again (alone) and use the whistle to lure Nessie. (see the related question below)
On your second trip to Loch ness, play darts to get the submarine ticket. Dive to the bottom where you will find a colorful "Nessie" head. Take the photo to the Pub, where it is revealed as a movie prop. But if you beat the original dart player, the bartender will give you the Pennywhistle. Take the rowboat out alone and you can use the pennywhistle to lure Nessie for a picture.