Travelling horizontally through or across water isn't too much of a problem. We have boats, depth strider, frost walker, speed buffs... Travelling vertically through is harder. The player currently only moves 0.2451m/s vertically underwater. Only 0.25 blocks a second? That's pretty slow.
Depth Riser is a variation of Depth Strider, also for boots. It goes up to level III at the exact same levels Depth Strider does. The highest chance in an enchantment table would be as follows:*
Depth Riser itself would simply make you rise and sink faster in water.
Depth Riser I: added vertical 0.5m/s (total of ~0.75m/s)
Depth Riser II: added vertical 1.0m/s (total of ~1.25m/s)
Depth Riser III: added vertical 1.5 m/s (total of ~1.75m/s)
Anything after level 3 itself would do nothing, so you couldn't just teleport to the surface or the sea floor without jump boost. Jump Boost does affect Depth Riser in the way Speed affects Depth Strider.
(level of jump boost/8+.46)+(level of depth riser no greater than 3*0.5) = total added underwater vertical speed.
As an example, at JB level 1 and Depth Riser III, you'd travel an extra 2.1 blocks per second, for a grand total of 2.35m/s underwater.
Depth Riser is not compatible with Frost Walker for pretty obvious reasons. It would be compatible with Depth Strider, so you'd be able to travel faster underwater in all directions.
I like the idea, but I would rather it be just rolled into Depth Strider. They are too similar to each other to be separate enchantments in my opinion.
The reason I'm suggesting a new enchantment and not expanded Depth Strider is because we have very similar situations already. Smite/Bane of Arthropods/Sharpness, Projectile protection/Blast protection/Fire protection/Protection... And to an extent Frost Walker and Depth Strider. They're all just variations (once again, to an extent Frost Walker) of the last in their list. It just seemed to fit in my mind.
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I support the concept, and think it'd be good if you increased your rate of sinking aswell as flying. Not as a curse, but to improve water mobility. I also think this should be wrapped into Depth Strider.
Whilst you're right that we do already have variants for protection and damage, they'd be used for different applications. People may want one set of blast prot armor for one expedition, fire prot for another. Same goes for the damage enchantments. In the overworld I take Sharpness with me to deal with everything as a general. If I'm going to the nether I take Smite in case I accidentally hit a pigman. I don't see a use where someone would want depth strider but not riser.