Here's level four:
Environmental effect: Everyone becomes more watery, gaining resistance to piercing damage, the ability to move through occupied squares with no penalty (even with enemies), and no penalties for squeezing.
The general idea of this level is a bunch of circular pools of different depths. The hallways are damp and have puddles in them, but are not underwater. Each room specifies the depth of the water, but the water level is always the same as the level of the floors in the hallways.
Room I: What a Shock
If characters come into this room from the waterfall from Level 3, Room XIII, they take no damage because the room is flooded to a depth of 10 feet. Above the level of the water are four doors. If the door knobs on these doors are touched, the character doing so takes 1d6 electrical damage, as does anyone touching them, as does anyone in the water if the toucher or anyone touching him is in the water.
The walls are smooth and are DC 20 to climb with a climbing kit, and impossible to climb without one.
Room II: Splish Splat
There is a shaft here leading up to the hatch in level 3 room XVIII. If someone falls from above without holding the hatch open, they take 3d6 falling damage. If the hatch is held open, the room floods. When the inflow stops, drains in the bottom of the room start to pump the water out. Everyone must make a DC 15 Strength save or be sucked against one of the drains, taking 1d6 bludgeoning damage for each of the three rounds the pumps are working. If you fail the save you may retake it after each round of damage.
Room III: Hey, Rusty
This room is filled with water to a height of about five feet. Any non-magical metal that comes in contact with it rusts, and gives a -2 penalty to attacks and skill checks. The door to the south has stairs leading out of the water to it. The other doors are at the level of the floor.
Room IV: Just a Bath
This room is filled with water to height of about three feet. The western door has stairs leading out of the water to it.
Room V: A Little More Than a Bath
This room is filled with water to a depth of about four feet. The northern door has stairs leading out of the water to it.
The secret door is below the water level and is DC 20 to find, DC 25 for creatures not adapted to seeing underwater. It has a DC 20 lock to open, or it can be opened by the thin blue key from level 2, room XVI.
Room VI: Myrmur
The water in this room is three feet deep. This can be considered difficult terrain for walking, if you are tall enough to walk in the room.
At the bottom of the water where the X's are there are two sets of plate mail, each with a trident next to it. If either of them is touched, the two water elemental myrmidons wearing them rise out of the water and attack with surprise.
There is a secret hatch in the floor on the northeastern side of the room. It is DC 15 to perceive, DC 20 for creatures who are not adapted to seeing underwater. It is locked, DC 18 to pick. It contains the myrmidons' high treasure.
Room VII: In the Pool
This room is empty, except for water to a depth of one foot. The water has a decidedly yellowish tint to it.
Room VIII: Wave Pool
This room is full of water to a depth of 4 feet, and the water is full of waves. It costs 15' of movement to move 5', and at the end of every move action you must make a DC 10 strength save or be moved in 5' in a random direction. If this would move you through a wall, you take 1d3 bludgeoning damage. Creatures with a swim speed have advantage on the saving throw.
Room IX: Inverse
This room is a pool 6 feet below the level of the doors. The water in the room is on the ceiling however, about 6 feet above the doors. If the word "water" is said by someone in this room, the water on the ceiling drops to the floor, doing 1d6 bludgeoning damage to everyone in the room. One minute after the trap is triggered, the water (and any creatures in it) fly back up to the ceiling. Then any creatures in the water fall back to the floor, taking another 1d6 damage.
Room X: Just a Chest, Honest
This room is full of water 3 feet deep, enough for difficult terrain. There is a chest in the middle of the room (under the water) with a high treasure and a lesser macguffin. There are also four water weirds in the room.
Room XI: Trap Door
This room is full of water to a depth of 3 feet. There is a 20 foot deep pit trap in the center of this room. It's DC 15 to see, 20 if your race is not adapted to seeing underwater. It is a DC 15 reflex save to avoid falling in, but due to it being full of water, you just float gently to the bottom. However, when you try to swim back up, you get hit with four +5 spear attacks from the walls. Each one does 1d8+2 damage if it hits.
Room XII: Big Trouble
This room is full of water to a depth of 40 feet. There is a stone house in the bottom of it. When the characters come in, a storm giant comes out and says he will give him a prize if they can stump him with a history question. If they do, they win a small treasure. If they don't, he tries to kill them. If they do kill him, he has a very large treasure.
Room XIII: Foreboding
This is the gym for the stone giant in room XII. The room is full of water to a depth of 30 feet. Against the east wall is a huge statue, scored with the marks of a giant greatsword and several lightning strikes.
Room XIV: Lobster Tails
This room is filled with water to a depth of 8 feet. It contains 6 giant crayfish. At the bottom there are the remains of other adventurers, with a medium treasure.
Room XV: Emptiness
This room is six feet below the door level, but has no water in it. Every time they search the room, they will find a new secret compartment that is empty. The old ones will disappear as they find new ones.
Room XVI: Hose off, eh?
The water in this room is frozen solid. Inside is a frost giant practicing ice hockey. His skates give him 50 movement, and his hockey stick has a blade on it, replacing the standard great axe. Any movement or combat requires a DC 15 check or the character falls prone, unless they have appropriate foot wear. There is no treasure in here.
Room XVII: Neutral Zone
This room is filled with two feet of water. The two dotted lines are walls of force. The activate when someone walks into the room from the east, and deactivate when someone walks into the room from the south.
Room XVIII: Hmmm Mmm
This room has water to a depth of one foot, just enough to keep the two chuul happy. Medium treasure.