I couldn’t find a good picture of this mob and so based on the description I thought it sounded a lot like the “Creature from the Black Lagoon” so thats how I converted it.
Medium elemental, chaotic evil
Armour Class 17 (natural armor)
Hit Points 229 (17d10 + 136)
Speed 30ft., swim 90ft.
STR 22 (+6) DEX 12 (+1) CON 26 (+8)
INT 18 (+4) WIS 17 (+3) CHA 11 (+0)
Saving Throws Dex +5, Wis +7, Cha +4
Damage Resistances acid, cold, lightning
Skills Perception +5
Senses blindsight 30ft., darkvision 120ft., passive Perception 15
Languages Abyssal, Aquan, Celestial, Infernal; telepathy 100 ft.
Challenge 11 (7,200 XP)
Blood Frenzy. The ghawwas has advantage on melee attack rolls against any creature that doesn’t have all its hit points.
Elemental Demise. If the ghawwas die s, its body disintegrates into a burst of water and foam, leaving behind only equipment the ghawwas was wearing or carrying.
Elemental Telepathy. The ghawwas can magically command any water elemental within 120 feet of it, using a limited telepathy.
Limited Amphibiousness. The ghawwas can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.
Rough Hide. Ghawwas have rough hides studded with jagged barbs and spiny protrusions. Any creature striking a ghawwas with a natural weapon or an unarmed strike takes 3 (1d6) points of slashing and piercing damage.
Multiattack. The ghawwas makes two melee attacks: one with its bite and one with its claws.
Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target.
Hit: 15 (2d8 + 6) piercing damage plus 4 (1d8) poison damage.
Claws. Melee Weapon Attack: +10 to hit, reach 5 ft., one target.
Hit: 13 (2d6 + 6) slashing damage plus 4 (1d8) poison damage.
Boiling Sea. A ghawwas can cause the waters around it to boil. Any creature within 50 feet of the ghawwas, within the same body of water, and at least half submerged takes 21 (6d6) points of fire damage (DC 16 Constitution save for half)..
When a marid gives itself over fully to its darker side of its nature it becomes this demonic creature. Bristling with spiny ridges, coral horns, and needle-sharp teeth, this menacing biped seems to have emerged from some poisoned sea.