The giant can breathe air and water.
The giant's innate spellcasting ability is Charisma (spell save DC 17). It can innately cast the following spells, requiring no material components:
At will: detect magic, feather fall, levitate, light
3/day each: control weather, water breathing
The giant makes two greatsword attacks.
Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 30 (6d6 + 9) slashing damage.
Ranged Weapon Attack: +14 to hit, range 60/240 ft., one target. Hit: 35 (4d12 + 9) bludgeoning damage.
The giant hurls a magical lightning bolt at a point it can see within 500 feet of it. Each creature within 10 feet of that point must make a DC 17 Dexterity saving throw, taking 54 (12d8) lightning damage on a failed save, or half as much damage on a successful one.