Amphibious. The giant can breathe air and water.
Multiattack. The giant makes two Battleaxe attacks and one Harpoon attack.
Battleaxe. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit:19 (3d8 + 6) slashing damage, or 22 (3d10 + 6) slashing damage if used with two hands, plus 7 (2d6) cold damage.
Harpoon. Ranged Weapon Attack: +7 to hit, range 50/200 ft., one creature. Hit:14 (2d10 + 3) piercing damage, and the target has the grappled condition (escape DC 16). While the target is grappled this way, its speed isn't reduced, but it can't move farther from the giant. The target takes 5 (1d10) slashing damage if it escapes from the grapple or if it tries and fails. The giant can grapple only one target at a time with its harpoon.
Reel In. The giant pulls the target grappled by its Harpoon up to 20 feet toward itself.
I learned enough about the Temple of Elemental Evil in my studies with Mordenkainen that I regard giants connected to the cult with a healthy amount of respect. When you combine a cultist's fanatical devotion with a giant's sheer power, the result can be horrifying.