You want to make a specific item, and you've described the theme, but you want input as to what to make it do? Essentially, anything you want!

Who owns this item? A Aquatic Elf Prince? A Temple to Deep Sashelas? A Rugged Sea Captain? Or is it unclaimed, lying at the end of a dungeon someplace? The ownership and/or history of the item is likely to influence what it does.

Are you the DM? If so, simply declare it exists, with any and all the abilities you want.

Are you a player? Than you're looking at Craft Arms and Armor feat, access to the appropriate spells, gold, xp and time, and so on.

But are you just asking for general suggestions for abilities? Here are some suggestions:

Least Trident of the Sea:
-Grant user Swim speed 30'.
- +8 to swim checks.
-No Armor Check penalty for movement in water.

Lesser Trident of the Sea:
same as above, plus-
-Breathe Underwater at will.
-Summon Monster III 3/day, Aquatic creatures only.

Trident of the Sea:
same as above, plus-
-Speak with animals, aquatic only, at will.
-Freedom of Movement always active.
-Control Weather 1/day.
-Cast all spells underwater without interference (like fireball underwater).

Greater Trident of the Sea:
same as above, plus-
-Charm Animal, Aquatic only, at will.
-Dominate Monster, Aquatic only, 3/day.
-Summon Monster VI 3/day, water elemental only.

Majestic Trident of the Sea:
same as above, plus-
-Plane Shift 1/day, anywhere aquatic. (Use it to show up in the ponds and sewer systems of layers of planes clearly not aquatic.)
-grants the wielder her own demiplane of water, accessible via Gate, at will, only accessible from the Material Plane.

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