The Blue Marine Zone boss is the fourth boss fought in Sonic Blast. It is an Egg Mobile-like combat submarine outfitted with a harpoon gun.
Upon Sonic or Knuckles entering the submerged, empty arena at the bottom of Act 3, Dr. Eggman will appear from the right side of the screen in his submarine, and open the compartment on the submarine's underside to prepare the harpoon gun and shoot a harpoon in his direction before retracting the gun and flying off the screen before coming back to shoot another harpoon in his direction. After this, he will fly right and left without shooting, albeit at a faster speed, making it difficult to hit him. When he comes back the fifth time, he will fire a harpoon directly towards the ground, then repeat his entire attack pattern from the beginning, albeit from the opposite side of the screen each time.
As the entire fight takes place underwater, the player is at risk of drowning if they go for too long without oxygen. Fortunately, an air bubble will be released every time the harpoon is fired at the ground. As soon as the air bubble appears, the player must grab it immediately to stay in the fight.
The mech's only vulnerable area is the glass windscreen, which must be hit eight times in order for the boss to be defeated; hitting it anywhere else will just rebound the player without dealing any damage. Defeating this boss will allow Sonic and Knuckles to finally reach Eggman's Silver Castle Zone.