Masters of the Deep Sea: GunDivers!
During the last battle, the Komusai II was knocked into the ocean. Captain, Shute, and the new GunDivers submerge to find it, and learn the secrets of the Dark Axis from it. Speaking of the Dark Axis, they're also underwater, looking for the Komusai II as well!
Aired 21/04/2004EP 16 - Masters of the Deep Sea: GunDivers!
2004
SD Gundam Force
The land of Neotopia is attacked by the Dark Axis, evil invaders from another dimension who wish to conquer this land. Neotopia's answer: the secret Super Dimensional Guard and their defenders, the Gundam Force. Led by Captain Gundam, the Gundam Force and its team of Gundams with special abilities are aided by a boy named Shute as they stop at nothing to defend the land from the Dark Axis and to defeat them once and for all.
(Source: Anime News Network)