This has already been added to the game as one of the easiest enchantments to get (you can get it on a helmet or book enchanted at level 1):
Aqua Affinity
Increases underwater mining rate.
Breaking blocks underwater is allowed at regular speed, though the player cannot be floating to get the full effect. Specifically, when swimming, mining takes 5X as long. Levels above 1 are no different.
Yup, with Aqua Affinity on your helmet and Depth Strider III on your boots you can mine very fast underwater. Adding Respiration III to the helmet will allow your air supply to last longer so you won't have to catch your breath as often or use potions. With that and a well enchanted shovel you can sprint underwater and mow down rows of gravel or sand.
So, recently I've been building with concrete, and obviously, I need gravel for that. However, even my god shovel isn't fast enough for my needs.
This isn't really a necessary addition, it would just be more convenient, something recent updates have been doing well (shulker boxes and elytra).
My suggestion is a 3-level "water haste" enchantment, named Glide.
It would be able to be equipped on shovels and pickaxes, since you're not really going to use an axe underwater.
I don't know the specifics on how fast digging would be with Glide I and II, but Glide III would have the same digging speed as out of water.
The enchantment would be obtained from Fishermen villagers or fishing, Glide III being around 20 or so emeralds (since it's only meant for 2 tools).
If you say something negative about my posts, I will eat your socks and steal all your iron nuggets.
This has already been added to the game as one of the easiest enchantments to get (you can get it on a helmet or book enchanted at level 1):
I see no need for a separate enchantment that does the same thing for tools; maybe tools can receive Aqua Affinity?
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
Wait, Aqua Affinity increases underwater mining speed?
I did not know that, apologies.
If you say something negative about my posts, I will eat your socks and steal all your iron nuggets.
Yup, with Aqua Affinity on your helmet and Depth Strider III on your boots you can mine very fast underwater. Adding Respiration III to the helmet will allow your air supply to last longer so you won't have to catch your breath as often or use potions. With that and a well enchanted shovel you can sprint underwater and mow down rows of gravel or sand.