Play underwater and collect as many coins and treasure as possible. Wait for your oxygen to reach 5% then resurfacing. Repeat the process as many times as needed. When you are underwater, various messages will appear. When a message is displayed, you cannot run out of oxygen, and the time spent in this state counts towards earning "Captain Nemo". Simply allow the game to idle while the message is displayed, then after enough time has elapsed return to the game, close the message, then either finish the mission or die due to oxygen deprivation. Note: The messages are less frequent if you are a higher level or have already completed the game. Start a new game to force a message to appear.
Easy "Paparazzi" and "Photo Sniper" achievements
Scroll over to the camera, then hover over a fish. Move around to completely cover the fish and wait until the number reaches 100% before taking the photo to get a high quality shot. Repeat this as many times as needed. You must purchase memory cards from the store to store the photos. Any photos that are not "high quality" will still use up one picture slot; avoid taking pictures that will just waste storage space.