To reach the Ocean Temple, read Carben's letter and tap the Rail Map icon below, then tap the Ocean Rail Map to draw the numbers from Carben's map onto your own map. Then get into your train and go to those places in order.
If you take the southernmost path, you will find Ferrus taking pictures along the side of the tracks. Stop next to him and he'll tell you that there are statues in this area that react to loud noises.
When you get to each numbered spot on the map, blow the train whistle. The red orbs in the fish statues will turn blue. After you activate the third statue, a giant statue head will come up from the water that you can ride the train into, so do that.
You'll find yourself in an underwater area. While underwater, your cannon can shoot homing missiles. You will occasionally get attacked by underwater Octoroks. The trick to hitting them is to tap them while they are moving, because otherwise the homing missiles will have a hard time catching up to them. If you don't kill them quickly enough, the Octorok will go in front of you in the track and attack you.
You'll also find floating Octomines. Just shoot these to get rupees and hearts.
When you get close to the Ocean Temple, an evil Armored Train will appear in front of it. Be careful, because it makes better decisions about which tracks to change to, making it harder for you to get away. You can't slow it down with your cannon, either. A Dark Train might appear on the tracks later, as well. If the Armored Train is chasing behind you, it can't actually catch up with you, so don't panic. Make your way into the Ocean Temple.
Go up and to the left. Read the signs and draw numbers on your map where each sign is. Then go up and past the arrow shooters in the wall, then go up the stairs.
To the left, there is a crack in the wall, but you can't go any farther down that path just yet, so back to the last room.
Go down and walk onto the large floating block. Face upward so you can block the arrows with your shield. Look at your map and copy the numbers that you wrote on the lower floor onto the map for the current floor. Then go down and step onto the block. Don't worry about the arrow shooters here: they only start shooting if you hit the switches in the wrong order. Use the boomerang to activate the switches in the proper order.
Step off of the block and go through the door. Don't go up to the crystal switch. Instead, stand near the small piece of fence and use the boomerang to activate the crystal switch. A trap door will open to your right, and the upper one will close, changing where the rocks are falling. Go back downstairs.
Go to the middle of the room, where boulders are now falling straight from the ceiling into the hole instead of blocking your path. Go north and head up the stairs.
Go past the yellow chuchus and head up the stairs to the right.
Go to the left and you must fight a Snapper. This guy will grab you with the whip. Turn and face the Snapper, then when he pulls you close, quickly attack with your sword. If you time it correctly, you will be able to hit the Snapper a bunch of times. Repeat this until the Snapper dies. Then you will receive the Whip.
Reach the Boss Door
Go downstairs for a moment and whip across the area at the bottom. To do this, tap the whip icon or hold R to keep the whip active. Tap the wooden pole, then when you get close to the next pole, tap it to move to it. On the other side, you will find a random treasure.
Next, go downstairs another floor and use the whip on the vines on the right side of the room to reach a chest containing a random treasure. Then go across the poles at the bottom of the room to reach a face switch in the wall. Use the whip on it to reveal a treasure chest at the north part of the room. Go up and open that chest to receive another random treasure.
Go upstairs and use a bomb on the crack in the wall, and also use a bomb on the wall to the right of the door. Go through the right side. Use the whip on the vines to get rid of the spikes, then go past. Use the boomerang to hit the crystal switch to the right.
Go out the way that you came in, then go through the crack to the left of the door. Watch where the floating block is on the map, then whip across when the block will be there for you to land on. Whip your way to the top of the room and use the whip on the face switch. Then whip back when the block is there for you to land on and go through the door to use the stamp station.
Now go back the way you came until you're in the area where you got the whip. Go to the left, whip across, and go upstairs.
The sign says to only pull the handle that is farthest away. Go down and whip across, go past the chuchus, then use the whip on the switch next to the door to open it.
Go up the shortest of the narrow paths: the one on the right. Use the whip on the wall switch. A bridge will appear. Go down it and go down the stairs.
There are two face switches to the left. Use the whip on the one on the left, then tap the switch on the right to move the sword into it. Open the treasure chest that appears to get a small key, then whip across using the new whip pole.
Go all the way north, whipping across. Use the whip on the pole to the right once the floating block is across from you, then open the chest there to get a random treasure. Then go back to the left side and use the small key to unlock the door. Go through and up the stairs.
Whip the switch to the left to open the door, creating a shortcut. Then whip across the poles to the right. Go down and you'll encounter some armored red chuchus. Just use the whip to pull off the armor and kill them with your sword. The doors will open. Go up the stairs.
Use the whip to pull the spikes off of the vines, then go past and read the sign. It says to remove the three swords in a row to open the path. Continue on and you will encounter two Snappers. Use the whip on them to stun them, then use the sword to kill them.
Go to the left middle part of the room, then go up and use the whip on the thing with the propeller on top. Grab the propeller with the whip, and be sure to keep the stylus touching the screen in order to keep from falling off. The propeller will take you to a ledge where you can go upstairs.
Go into the small room and use the whip on the face to pull out the sword. Use the sword on the chuchus to kill them. This will reveal a whip pole on the right side of the room. Go back downstairs and use the propeller to go back to the main area.
At the left middle part of the room, there are three statues in a row. Use the whip to pull out all three swords. Then go to the top area of the room and whip your way across.
In the upstairs room, whip across. To reach the rightmost whip pole, you will have to fall and then tap the pole. Just tap the pole until you fall and go across. Then go down and read the sign, which says to make a note of the positions of the swords.
Go to the left middle part of the room and remove the spikes from the vines, then activate the face switch up above to reveal a chest. It contains a random treasure. If you used the propeller on the lower floor, you can get the small key from the right side of the room.
Now go back the way you came, activating the crystal switch to change the position of the whip pole so you can whip across and go back downstairs.
If you didn't use the propeller on the left side of the room yet, do that so you can get the small key from the right side of the room floor 6. Get that key, then return to floor 5.
Now go around and change the sword positions to match the ones on the upper floor. The door in the bottom right area will open. Use the key on the door at the bottom of the room (see above for how to get the key) and whip across and go upstairs.
Use the propeller to land on the floating block, then press the switch next to the Big Key. Pick up the big key and walk across the bridge. As soon as you cross, four Key Masters appear. Put the key down and kill the Key Masters, then quickly pick up the key and run up and use it on the big lock. The Key Masters will disappear. Go upstairs and you will fight Phytops.