So, oceans are pretty dull right now. Right? But I was thinking the other day, oceans are a large open area. Perfect for a boss. So I started thinking and came up with the whale.
The whale is more of a mini boss than a boss, as they can spawn in the same area more than once. It is a moderately rare spawn in oceans, is rather large, and a maximum of four can exist at one time though it's rare enough that you'd be lucky to see two at one time. Let alone three or four. Whales are non hostile unless provoked, and they have three attacks.
The first one occurs when you are in front of it. It opens its mouth slightly and sucks in water, mobs, and you. This attack deals five hearts unarmoured. The second is a tail attack. It hits you with its tail, knocking you back significantly, and dealing seven hearts of damage unarmoured. The third one is that it will try to get underneath you, and it will spray water from its blowhole. This deals one heart of damage per second you're in the spray, and armour does not protect against this attack. It also sends you flying upwards.
The whale has 100 hearts of health (by comparison, an iron golem has 50, and the Ender Dragon has two hundred). Hitting its belly will deal slightly more damage than a normal hit.
When killed, it drops 250 xp (brings you up to level 10 from nothing) and 30-40 blubber. Blubber has a few uses, and all the uses are useless unless you have a decent amount of it.
The first use is that blubber can be used to smelt twelve items, the same as blaze rods. But as blaze rods are considerably easier to get, it's just better to use blubber for its other purposes.
Blubber can be placed on the ground like redstone, and is flammable when placed as such. Fire spreads on blubber very quickly, allowing it to be used as a very effective fuse. Also, when placed on the ground, blubber is slippery. Less so than ice, but slippery enough that it makes fast item transport viable without ice. Unfortunately, blubber is washed away by water, so you have to get creative with blubber placement for fast item transport.