Minecraft wiki entry for Combat, associated text: Loyalty is the best enchantment for a trident as it will make thrown tridents come back to their user, which is useful when using it as a ranged weapon.
Impaling is a good enchantment for hunting guardians or elder guardians, as it deals high damage to aquatic mobs in Java Edition, while in Bedrock Edition it deals extra damage to any mobs which are in contact with water or rain, including players.
Channeling is a good enchantment for fighting during thunderstorms. As this enchantment will summon a lightning bolt if it hits a mob during a thunderstorm, causing both contact damage as well as additional damage from the lightning bolt and from setting the target on fire. One should be careful, however, as lighting can potentially cause a variety of bad things to happen such as causing villagers to turn into witches, pigs into zombified piglins, creepers to become charged, destroying dropped items either by lightning or spawned fire, and if on high difficulty, sometimes spawn a group of skeleton horsemen if a skeleton trap is triggered.
Riptide is a good enchantment if you want to travel long distances. It shoots you up out the water, and if it's raining, then you can shoot around in the rain like a propeller. It also deals splash damage.‌[Bedrock Edition only]