Undersea Dream takes place in the Dream Depot, a special world that contains the dreams of all creatures and beings. The board is full of seashells, coral reefs, and underwater Mario characters. There are numerous houses made out of seashells and shops that are owned by Cheep-Cheeps. There is also a sunken ship located at the top left hand corner of the board. Mario and the other characters will be able to breath and move in the water, as well as the non-playable characters. The characters will be able to interact with the undersea wildlife while navigating through the board. Undersea Dream does not house any gimmicks or special characters; it is basically a straightforward board. The only type of events the player will encounter are happening events. At the top of the board is a statue of a whale. If a character lands on the happening space in front of the statue, they will be transported to the space before the Star Space. There is also a shop that is owned by a Cheep Cheep at the bottom of the board, where the characters will have a chance to win a capsule. At the bottom of the boards, there are two happening spaces that are opposite from each other on the board map. If a character lands on one of the spaces, they will have the option to ride a Manta Ray to the other side of the board and gain coins along the way.
- Manta Ray Ride - At the bottom of the board, there will be two logs with a happening space in front of them. When a character lands on one of these spaces, they will have the option to ride the Manta Ray to the opposite side of the board. If the character accepts the offer, they will be able to ride the Manta to the other side. Along the way, they will be able to collect coins by controlling the Manta.
- Dolphin Ride - At the top of the board on the plundered ships window, there will be a picture of a Dolphin on it. If a character lands on the happening space near the window, a Dolphin will appear out of it and transport the character to the other side of the board. They will be transported to the area with the giant clam shell.
- Cheep-Cheep Shop - At the bottom of the board, there will be a shop that contains a Cheep Cheep and a happening space in front of it. If a character lands on the space in front of the shop, Cheep Cheep will ask the character to select one of the three pearls. One of the pearls contains a capsule, while the other two contains an opened can and an old boot. If a character selects the pearl with the capsule, he/she will be able to keep it.
- Whale Statue - At the top of the board near the Capsule Machine, there will be a statue of a whale with a happening space in front of it. If a character lands on the happening space, they will be asked if they would like to ride the whale. If they choose yes, the whale will transport the character one space before the Star Space. However, it has a small chance of boosting them after the space.
- This is the only board map in Mario Party 5 that contains the least number of capsule machines: 2.