You've arrived at Deep Darkness. The first thing you'll want to do is talk to the tropical bird to save. The monkey nearby let's you rest up, restoring your HP and PP. There is also another monkey that will ask you teach it to teleport. Do so and it will give you the Monkey's Love item. If you don't have it in your inventory, call Escargo Express and have them deliver the Hawk Eye to you. When you venture into the water, make sure you stay above water as much as possible. The deep water will slowly drain your HP while making you move very slowly.
Head to the right but go south as soon as you enter the water. In the deep water near the bottom left tree that is above a log, you can find a Magic Truffle. Use your Piggy Nose for help in finding it (it doesn't show up on the screen). As you move to the right, you'll find an Arms Dealer. Buy Jeff some Multi-Bottle Rockets if you have the money for it. To the north of the Arms Dealer is a single tree. There is another hidden Magic Truffle near that tree.
Head south of the Arms Dealer and you'll enter an area that is pitch black. Use your Hawk Eye to light up Deep Darkness for the rest of the game. On the way south, pick up the Banana from the present. As you head farther south you'll notice a pipe-like object sticking out of the water. This is actually a man who acts as an ATM machine. Unfortunately he'll charge you a fee for using him (you'll end up paying the same amount to him as you withdraw) so if you really need to money just teleport somewhere else for it.
The enemies you encounter here are fairly easy. You can outrun most of them and the ones you can't are relatively weak. Do be careful around the oaks because they (like their easier cousins) explode at the end of battle. Countinue south until you reach land and head east. Go north and to the left getting out of the deep water as soon as possible. There is a present with Beef Jerky farther north and another with an IQ Capsule to the west. In between the two presents next to the trees to the north there is another hidden Magic Truffle. Head south from here and you'll enter deep water again.
Follow the path around and you'll come upon a damaged helicopter. Could it be the one Pokey stole? Just north of it you'll find a Cup of Lifenoodles. There is a lot of deep water ahead so make sure you replenish some HP before moving on. There is a hidden Magic Truffle along the northern tree line at the very end of this deep water. Go to the left and take the south-eastern split to receive a Souvenir Coin. Then head back and take the northern split. Go straight north when have the option of going west, north, or east. This will lead to a Rock Candy (remember the duplicate items trick?). Go back to the intersection and head east to come upon a boss.
For winning the battle, you will receive the Casey Bat, a very powerful bat for Ness. While it is strong and causes you to SMAAAASH! more often, it misses quite a bit so there is a trade-off. It is up to you to use it or not. Poo can now use the Starstorm spell during battle as well. After the battle follow the north and to the left. When you reach shallow water again, head straight north from the tip of the light blue water and you'll find the last hidden Magic Truffle. Head to the left and enter the cave.