Its been a while since we have seen a boss added to the game, We had the ender dragon then the wither and then the elder guardian. Why not add an ocean boss that is summon-able?
Something along the lines of a Kraken, you have to fire arrows at its tentacles to take them out (like the crystals for the ender dragon) but with health.
Taking out the tentacles removes the Kraken's main bodied shield so you can attack it.
The attacks would come in the form of swipes from the tentacles and ink shots that cause blindness for a few moments.
Ink shots would only cause blindness while a hit from one of the tentacles would cause damage (range depending on difficulty level
The Kraken could only be summoned by a depth charge thrown into the ocean over a ravine.
Depth charge would be made by 4 wood planks of any type in the corners of the crafting bench, one heart of the ocean in center, 2 tnt on top/bottom
and two obsidian on left & right side. once the item hits the water it starts to sink down and flashes like tnt until it gets 5 blocks above the ocean floor. Once it explodes the Kraken would spawn and move towards you.
Defeating the Kraken would give a trophy in the form of a head (like the dragon)
Large amount of Exp, A "cutlass" sword with random enchantment(s) - just a different design of the regular sword except it has +2 attack speed? maybe? and a Pirate hat. - cosmetic item ( can be enchanted) - I really think we need to have themed cosmetic items we can earn, we have capes! why not?!
The item rewards could come out of a chest down in the ravine where you spawned the boss or just dropped by the boss itself