The Sea

The sea is part of Pokémon Mystery Universe's overworld, and it connects the regions together. All regions excluding Winden have mapped sea areas. Some portions of the sea are inaccessible due to rock barriers. The sea can be accessed at shores that are placed throughout the regions, and requires a way to traverse though water, including: Most of Archford's sea is accessible due to it being an island.There is also no real purpose to traversing the various sea maps other than if you wish to view certain parts of a map that couldn't be seen otherwise. Archford has dive spots on a few of its sea maps, that lead to and come from its underwater area.
 * Water-types
 * Ghost-types
 * Flying-types
 * Levitate
 * Using the move Surf
 * Holding a Float Stone, Air Balloon, or Mobile Scarf

Trivia

 * While players were able to travel on water in dungeons, players were not able to travel on the sea until May 5th 2014.
 * The sea used to let players access dungeons that would otherwise be unavailable until a much later point in time.
 * When the sea was first released there was one location that lead to an unreleased map. This map was PvP and players could kill each other inside of there.
 * It was possible to travel to Tanren using the sea without obtaining any HM.