Homebrew
Coral Weaponry. The gilman's melee weapon attacks do not gain disadvantage when submerged in water. Limited Amphibiousness. The gilman can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.
Multiattack. The gilman makes two melee attacks: one with its bite and one with its claws or trident. Bite. Melee Weapon Attack: +4 to hit, reach 5ft, one target. Hit: 4 (1d4+2) 1d4+2 piercing damage. Claws. Melee Weapon Attack: +4 to hit, reach 5ft, one target. Hit: 4 (1d4+2) 1d4+2 slashing damage. Trident. Melee or Ranged Weapon Attack: +4 to hit, reach 5ft or range 20/60, one target. Hit: 5 (1d6+2) 1d6+2 piercing damage, or 6 (1d8+2) 1d8+2 piercing damage if used with two hands to make a melee attack. Larva. If the gilman is carrying larva (if and how many should be determined when building the encounter), it may throw one as an action. The gilman makes a ranged weapon attack against the target (+2 to hit, range 20/60, one target). On a hit, the larva lands within 5ft and immediately succeeds on a attack with its tentacles using its reaction. On a miss, the larva still lands within 5ft of the target, but no attack is made. The larva takes its turns immediately after the gilman.