Adult sea dragon

Amphibious. the sea dragon can breath air and water.

Spellcasting. The Adult sea dragon is a 6th-level spellcaster. Its spellcasting ability is wisdom (spell save DC 8, +2 to hit with spell attacks). The Adult sea dragon has following  spells prepared.

Cantrips (at will): frostbite, primal savagery, shape water, ray of frost, druidcraft.
1st level : absorb elements, chaos bolt, ice knife, create or destroy water, purify food and drink, animal friendship, speak with animals, arms of hadar.

2nd level : dragon's breath, snilloc's snowball swarm, animal messenger.

3rd level : tidal wave, wall of water, wall of sand, create food and water, water walk, conjure animals, water breathing, hunger of hadar.

4th level : elemental bane, watery sphere, control water, ice storm, evard's black tentacles, conjure minor elementals.

5th level : conjure elemental, cone of cold.

6th level : investiture of ice, otiluke's freezing sphere, wall of ice.

some adult sea dragons choose the path of sea storms witch gives them the following spells.

control winds, investiture of wind, maelstrom, storm sphere, steal wind strike, thunderclap, thunder step, zephyr strike, whirlwind, call lightning, chain lightning, control weather, gust of wind, gust, warding wind, lightning bolt, tsunami, wind wall, witch bolt.

Multiattack. the Adult sea dragon makes three attacks: one with its bite and two with its claws.

Bite. melee weapon attack : +9 to hit, reach 10 ft., one target. hit : 2d10 piercing damage.

Claw. melee weapon attack : +9 to hit, reach 5 ft., one target. hit : 2d6 slashing damage.

Tail. melee weapon attack : +9 to hit, reach 15 ft., one target. hit : 3d8 bludgeoning damage.

Tidal wave breath. the dragon exhales a torrent of deadly water in a 90 foot line that is 5 feet wide. each creature in that line must make a dex saving throw of 16 or higher taking 5d10 water damage on a failed save.

Frost breath. the dragon exhales an icy blast in a 60 foot cone. each creature in that area must make a con saving throw of 15 or higher taking 4d12 cold damage on a failed save.

Lightning breath. the dragon exhales lightning in a 90 foot line that is 5 feet wide. each creature in that line must make a dex saving throw of 16 or higher taking 5d8 lightning damage on a failed save.

the Adult sea dragon can take three legendary actions.

water spout. 30 feet two targets.  the dragon roars at a point in range and a water spout erupts targets must succeed on a dex saving throw if fail they take 3d6 water damage + 1d10 fire damage.

whirlpool. 20 feet three targets.  the dragon stares at a point in range and the targets must succeed on a dex saving throw if fail targets get prone in the whirlpool taking 4d12 bludgeoning damage.