The First 15 Minutes of Dolphin Blue (Dreamcast)

Copied from MobyGames:

Dolphin Blue is a 2D side-scrolling run'n'gun game similar to the Metal Slug series and Sammy's flagship run n' gun game for the Atomiswave arcade board.

Almost all land has disappeared underwater in the future however humanity has managed to survive and established new nations on the limited dryland that remains. In the game the player takes on the role of either one of two Royal army soldiers Eiro and Ann and their dolphin who have to fight through the post-greenhouse flood land to stop a force known as the Evil empire from taking over the world.

The gameplay is similar to the Metal Slug series, with weapons firing in eight directions. However players may use their Dolphin to launch key special attacks. Stages take place both on land and underwater. The game also has fewer platforming segments than many other run 'n gun games and can be played solo or with a friend.