You can find all of these Infos also inside the manual to the game. But if you have lost it (or got other reasons why you don't have it), you can read about them here.
You can enter a [PARENT'S OPTIONS] screen by pressing the buttons [A], [B] and [C] together at the title screen. There are several options that can change the actual game by adding more stages, also secret ones. To get the games full potential, set the game [DIFFICULTY] to [HARD]. Only then you can play all 27 stages ([EASY] contains 11 stages, [NORMAL] 17).
There are also secret stages, to find these set additionally to hard difficulty the [MODE] to [EXPLORATORY].
These were taken from the game's manual. Table uses following format:
Stage : Password
Home Sea : CSJ
Aqua Maze : CFR
Bay of Songs : HOF
Seal Rocks : RCL
The Sea of Music : JHO
Octopus Passage : ROC
The Enchanted Sea : OSH
Mysterious Ride : HRL
Vents of Pearl : JRF
Treasure Caves : SLR
Ocean Tag : FCJ
The Fish Caves : CRS
Turtle Islands : LJS
The Lagoon of Stonefish : FSL
Dolphin Ride : OFJ
Seahorse Reef : LOH
Melodic Waters : FLR
Fish Reef : FJF
The Ocean of Mimicry : CCJ
The Lagoon of Songs : JLL
The House of the Hermit Crab : HRR
Shark Sea : SCS
The Magical Ocean : LSL
The Bay of Scattered Song : SSR
Sheel Roundup : CRS
Meeting Maze : RHJ