From Bulbapedia, the community-driven Pokémon encyclopedia.
The Beach (Japanese: かいがん Beach) is an area in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness and Pokémon Mystery Dungeon: Explorers of Sky, located southwest of Treasure Town. It is accessed via the crossroads.
The beach serves as an especially important location throughout the story of the game. In the beginning of the game, it is the location where the player meets the partner, and is where the story of a human transformed into a Pokémon began. The partner meets the player during the event of Krabby blowing bubbles. This will also happen when the player returns after he/she disappears after the initial part of the storyline.
Also found at the beach is an entrance to the first dungeon in the game, Beach Cave, where a recurring side antagonist, Team Skull, is found for the first time, although missing their leader Skuntank. In addition, Lapras returns to the shore in order to grant access to the Hidden Land once the team has gone Brine Cave, and remains there for the rest of the game.
Although not necessarily an important moment, Team Raider can also be found here at the end of the whole storyline, having saved Dugtrio, Diglett's father, who also is a sea lover, from "drowning". This "drowning" is actually a splash of water touching the triple headed character.
In Explorers of Sky, bottle mail can also be found here. This is the only way to obtain treasure memos.