The original Super Mario Bros. introduced the idea of an underwater sub areas, spanning no more than four screens in length. It’s an idea worth using if you would like to make an underwater area, but do not want to design an entire underwater level.
Super Mario Bros. shared the exact same underwater area in both World 5-2 and World 6-2. The enemy roster included three gray Cheep-Cheeps, one red Cheep-Cheep, and two Bloopers. Hazards were minimal, especially compared to a Hammer Bro. aboveground in 5-2. The drawback is that taking the sub area meant missing the coin-filled cloud bonus area, as the original Super Mario titles did not allow backtracking.
Bowser’s castle in World 8-4 presented a mostly empty, but unavoidable, underwater area. Here, five fire bars and three Bloopers test the player’s skill.
The Lost Levels introduced two more underwater sub areas, across three levels.
World 4-1 provided an underwater sub area filled with enough coins that it feels more like a typical bonus sub area, save for the three Bloopers and two Buzzy Beetles moving around.
The coins are hopefully rewarding, because the exit pipe takes the player roughly a mere 10 blocks away from where they left the main area.
In the shared design used in World 6-1 and World 8-1 underwater sub areas, once again the player is met with many coins. These underwater sub areas are light on enemies, with two Bloopers, a gray Koopa Troopa, and a red Koopa Paratroopa.
The former sub area allows the player to skip over an uneventful three screens in the main area, but the latter curses the player by dropping them at a warp zone back to World 5. (Let’s just wait for the timer to run out.)
As shown in Super Mario Bros. and The Lost Levels, all a Super Mario Bros. style underwater sub area needs is to be a simple area, nothing too difficult, but maybe a bit rewarding.