This spell requires a magical conch shell taken from the sea- Ask a question into the magic conch. The DM will roll a d20 and record the number. Then, the DM will give a vague sounding answer based on the roll; a roll of 1 might be "never" and a roll of 20 might be "yes and better then you think", where as a roll of 10 might be "maybe", and so on.
If you act upon the question of the magic conch, the DM will substitute that roll with a roll made by yourself or a creature. For instance, if you ask the magic counch "will I die before I leave this dungeon?" and the answer was "no" because the roll was a 1, then the DM could replace that roll of 1 the next time an orc tries to attack you, and so on. This is a double edged sword, as if you asked "will I survive this dungeon" you could just as easily get a roll of 1 or 2 which the DM can use to substitute any saving throw you make with something that will kill you instead.
This divination of fate isn't fullproof. If circumstances or the players themselves break the chain of events that would contradict the magic conch, it shatters apart.
 Stairway in the Waves
Rising from the ocean; crystal stairs rise up upon casting this spell. This spell can only be cast at night, as concentrated light (from a lantern, torch, or sunlight) will break the fragile enchantment. This spell makes crystal stairs appear from the water- they are physically present but not strong enough to cause damage to ships or large animals moving through water. They are fixed in place; though you could be thrown off by a strong enough wave or crash of water.
The stairway can be spiral (maximum of 60 ft of height), ascending in one direction, a curve, etc. Steps can also be wide, steep, shallow, whatever. Night sirens use this spell to lounge around above the water and entice sailors, or give them a way to descend from the deck of a high ship down to the random precarious rock in the middle of the ocean they're always lounging on.
 Magic Bubble
Surrounds a person in a bubble that can float around. The bubble is under the control of the caster, and can envelop one human-ish sized creature, or a handful of smaller creatures. The bubble can float in any direction, including underwater, and moves at a brisk walking speed. It is partially cut resistant, but can be popped with any sharp object- consider its AC 18 for an attack roll to pop it, with ranged or melee inside the bubble. The bubble could not survive being moved through a coral reef or forest; it can only go through tight passages if there is ample room.
This spell can be cast on a friend or foe, both can be useful. The bubble also traps whatever is around the creature at the time, so aquatic creatures keep an amount of water for breathable medium or a sphere of air is kept for air-breathing creatures who are taken underwater or into space, etc. The bubble technically lasts forever until it is popped, which really won't take that long.
 Beguiling Barnacle
It requires touch and a willing or relaxed target to place this barnacle on them. Usually, the forehead or one of the hands is chosen for this spell.
The magic barnacle sticks to the target and acts as a Charm and Quest spell. Those under its effects are in love with the caster, and will go to the ends of the Earth to accomplish whatever task they are given. If a Remove Curse or they are 'snapped out of it', the barnacle remains, squeezing and causing them pain the longer they resist the command. The barnacle can also be pried off with a successful save and reduces the victim's health by -1d4 permanently.
As an added effect, anyone with one of these barnacles on their skin absolutely cannot swim.
 Distortion Wave
Requires both hands to be touching a body of liquid- both hands shove out a magical wave. The wave has two effects; the first is a powerful splash- not enough to harm, but enough to shove people back. This disrupting casting a spell. The second is the disorienting effect this has on anything swimming near the surface; divers, fish, monsters, etc. They get spun around and confused, taking a round to reorient themselves.
Mermaids usually use this spell in surprise when some pirate is swimming after them but has stuffed his ears full of wax or cotton.
This spell is pretty strong. Anyone who casts it will take 2d6 magical damage, nonlethal. This spell creates a magical underwater tunnel, cave system, or base underneath an island. All of the entrances to it must be underwater, but it can have as many as you want or as few as one. The underisle is hollowed out and magically formed from the rock, and has no bearing on the structural stability of the island, nor does the displaced water cause any visible movement besides some bubbles and surprised fish.
The island's size and shape generally infer the size and shape of the underisle. If the island is small enough to fit on a gridmap, then it has the same number of grid spaces for the underisle, in any combination of rooms and passages of any shape that the caster desires. If the island is big enough to fit on a hexmap, then you can only dig out a small amount of space in a single hex. Total space of about 200 squares per area seems fair.
The Underisle is not furnished beyond with stone. If you press your ear to the roof of the underwater chamber, you might be able to hear footsteps on the island above. You could also dig up or somebody could dig down to connect the isle with the underisle. While the Underisle will not collapse naturally, it combined with an earthquake or some big bombs could cause the entire island to collapse in on itself, or cause a small inlet bay to form if used on the mainland. This is in fact how the mermaids intend to retake the surface world and make the world one big ocean again.
The Whirlpool forms as a small sucking circle of water in any body of water- anything as small as a bathtub or as big as the ocean. Every round the speaker chants the spell, the increases in size by one "square" or size unit. This must be continued- the moment the caster is struck or interrupted, the spell ends and the whirlpond quickly dies down. Anything caught in the whirlpool is sucked towards the center until they are then brought under- which means they sink to the bottom of the body of water in an instant. Large objects like ships or giants are only sucked under if the whirlpool is large enough to accommodate them.
Naturally, this is the main method that sea witches and ocean casters use to sink ships of surface dwellers. Protecting oneself from archers while standing on the shore and casting this spell to defeat an invading navy is a surefire way to become the king's court magician in any nation. However, those who steal these most sacred enchantments from the sea are said to be doomed to return them by watery grave.
 Conveniently Clothed
This is the reason why mermaids always appear with little shells over their boobs instead of just being topless, as you'd imagine a bunch of weird sea creatures would be. When cast, this spell causes random items in the area; small living creatures, trash, plants, and so on to become stuck onto the caster in convenient ways to censor their nudity to all who may be leering. If you cast this in a crowded market on someone naked; within a moment they'd find themselves covered with an empty coinpurse, some flyers from the faire, and a banana peel over their junk. In a jungle? Convenient leaves are slapped on by the wind, and so on.
Unlike the other spells on this list, there is no known practical reason as to why mermaids cast this spell on themselves. As they are not human, and do not need protection from the weather or other elements. They have no civilization to speak of- so what purpose does this serve?