Basically, the Aquatic Update took a look at Mariculture, and asked 'Can we screw this up as badly as we did EnderChests? Well, let's find out'.
Not sure if I agree with that.
They now made it so that stairs and slabs look nice when combined with water, or at least tried as there are some render glitches that are most likely going to be patched. Fences should follow and so should signs if they don't already behave the same.
Placing soulsand underwater make you rise in it very fast, I believe items react the same which now finally gives vanilla players a good way to transport items upwards without using bugs or use a lot of redstone and droppers. You also technically don't need soul sand to transport items up in water as they float up anyway now, its just nice to speed them up.
Those magma block (whatever they are called) do the opposite which may prove useful for vanilla players, or not. We will have to see on that.
Because items now float in water they move a lot faster in water streams, again something very useful for vanilla players. This may also mean that players will spam less hoppers as waterstreams are now defiantly faster even without ice making them very attractive to work with again, even if they take up a bit more space.
There is now a proper swimming animation, and I believe you can move faster while swimming.
As for the trident, it seems like a very versatile weapon with all the enchantments that it can have that alter behavior together with the fact that you can use it both as a ranged weapon and a melee one.
Turtles give vanilla players a way to have constant water breathing, again something they wouldn't have otherwise and will thus defiantly help them with this update. They are also kind of complicated in how they behave, with them laying real eggs that hatch on their own (thus not just items like chickens do) so maybe we will see some more interesting mob behavior in the future now that Mojang has at least build something for turtles.
As for enderchest, you shouldn't compare the two. One is an expensive backpack the other teleports items. The one thing they share is a name, but everything else is different about them. As for why Mojang decided to make an backpack. Simple, vanilla has less items then modded and thus less reason for one. If you are at the point of needing one you are probably a bit further into the game and can probably afford one. You also don't need to go to the nether for one, so it is technically cheaper then an ender backpack.