Red Glowing Statue Underwater Causing Damage?

These statues are called Cursed Mermaids. They are fairly common around the shores of islands. If you destroy them, they drop a Mermaid Gem which you can sell back at an outpost for a relatively large amount of gold when compared to treasure chest hunting.

Destroying the mermaid statues is quite difficult. It is suggested you have three or more players to destroy the red statue but it is possible with two. If you are playing alone, it is impossible to destroy them. When you shoot one, they begin to restore health right away. You need to unleash a constant stream of damage to take them out before they heal.

It is best if you do this from your boat. Move the boat using the harpoon so you can shoot the statue while standing close to the ammo crate. Both of you equip a flintlock and a sniper. Unleash all the shots you can as quickly as possible. It will eventually be destroyed but it does take some time.