This table generates sea monsters suited for attacking a party on a ship. I mean, who goes on a ship in D&D expecting good things?
Another way to use the table, and a good way to surprise the party, is to have the monster attack a fishing village from the sea, roam around a coastal fortress, or swim upstream on a river. Or perhaps in a tank in a dungeon?
The monsters in the table will have a lair, tactics and some very loose behavior structure, so they make for good persistent threats.
The monsters here are not perfectly suited for true underwater adventures. They can be dropped in, but some of the tactics may have to be tweaked. These creatures are better suited to attack ships or people.
|1||A flathead fish made of coral and algae.||Can shapeshift between its main form and a bank of small fish.||Its bite is exceptionally strong and corrosive.||Will prioritize food over anything. Can be lured by bait.||Banks of sand emerging from shallow waters. The banks were put in place by people worshipping the creature.|
|2||A jellyfish made of viscous black ink.||Stalks the prey for several miles waiting for the best moment to strike.||Knowingly targets ship hulls.||Is especially vulnerable to magical damage.||A wrecked trading sailboat with crates of acid potions of onboard.|
|3||A 100 feet wide sole fish||Lies at the seabottom and suddenly strikes. Uses camouflage.||Can devise and lay traps both at depth and on the surface. Cunning, scheming.||Its belly is much softer and more vulnerable than its back.||A rock formation with a cave system and underwater lava streams|
|4||A tortoise that can breathe fire.||Hunts in a pack.||Has a head that has rock consistency – can ram small ships and will resist piercing damage.||Relatively low intelligence creature – will attack even large groups head on.||An underwater area covered in tropical vegetation due to the presence of a magical orb at its center|
|5||A whale made of slab stones.||Has enslaved a group of pirates to procure food.||Can cast charm spells and the like. Can disappear and reappear suddenly.||Cannot survive in cold temperatures or waters.||An ancient underwater stone temple built by a long-gone civilization.|
|6||A kraken punctured by several harpoons and swords.||Is not prone to foraying out and attack ships but is extremely territorial around its lair.||Has extraordinary perception, equivalent to true sight.||Will be distracted by shiny objects.||The creature lives inside an ancient giant blue whale in a symbiotic relationship.|
|7||An amalgam of barrels dumped by ships held together by a dark elemental force.||Is not often seen, but migrates every year at a known time along an ever-shifting route.||Can use its tongue, tendrils, or a tool to pull people into the water.||Having sustained previous wounds, is afraid of harpoons.||A forest at the bottom of the ocean.|
|8||A sea spider that can swim and crawl up a ship’s hull||Is in constant communication with an intelligent being controlling it and using it to spy.||Has swallowed a wizard and absorbed its powers – can cast like a 3rd level magic user.||Has to resurface every minute.||The creature takes over the harbor waters of coastal towns and feeds on water waste.|
|9||A ghost of a dead sailor with a small ghost ship.||Feels a responsibility towards fish and will show up to protect them.||Communicates through songs with other member of its speacies and alerts them. The songs can cause psichic damage to humanoids||Is hated by the gods, and clerics can use this to their advantage.||A coral reef growing on a ley line or another powerful source of natural magic.|
|10||A weretuna. This large giant tuna turns into a boodthirsty killer at night. Its yellow fins are made of solid gold. What a sight!||Moves at a great speed just below the water surface, often ripping through nets and escaping undetected.||Can summon small storms.||Though powerful, it is slow moving.||An abandoned mussel farm that drifted out to sea.|