Swimming - Monster Hunter Tri
After completing the Wyverian Artisan's quest, speak to the Village Chief and then cross the bridge and exit to Moga Woods.
You must hunt one Molid to complete this quest. Molid are large, harmless fish. You can find these creatures in Area 11. You must swim through Area 10 to get to Area 11. Time for some swimming lessons.
Monster Hunter Tri adds swimming and underwater combat to the series.
Combat underwater is much like combat on land, except underwater you can move vertically as well. Swimming underwater requires using the Left Stick and the camera controls together. To swim, you simply point the camera in the direction you wish to travel and push the Left Stick forward to swim there. This takes some getting used to, especially if you tend to use the L button to centre the camera behind your hunter rather than controlling the camera manually. You can also press the ZL button to ascend and the ZR button to descend, but only while you're not moving.
The Oxygen Gauge on the left side of the screen will steadily decrease as you remain underwater. To refill it, return to the surface or find an Air Supply Point, which appear as a stream of bubbles on the seafloor. There are also some items that can restore the Oxygen Gauge. That said, your hunter can hold his or her breath for a ridiculously long amount of time, so there's little need to worry about drowning, provided you keep an eye on the Oxygen Gauge.
Swim through Area 10 to Area 11. A video sequence will point out the Molid. You have been given three Fishing Harpoons; killing a Molid with a Fishing Harpoon rather than your weapon will allow you to carve the dead fish.
Kill at least one of the Molid in Area 11.
You just need to kill one Molid to complete the quest, so once you've done that, if there aren't any additional materials you'd like to gather while you're out here, press Start and select the “Return to Village” option from the Hunter menu to warp back to Moga Village.