Describe the bug
I put mending enchants on gear from better diving, and it has no effect, items are not repaired.
Especially annoying if there is no other way to repair items and drowned are very common to spawn in water

To Reproduce
Steps to reproduce the behavior:

1. Get yourself diving set
2. Enchant with Mending
3. Get damaged
4. Cry as damage is irreversible

Versions
Other mods (that are also installed): From FTB Oceanblock 1.5.0, originally opened issue there
FTBTeam/FTB-Modpack-Issues#204

Mending is working fine for me. This is either a mod conflict or you might not fully understand how mending works.

This is the description from the minecraft wiki.

Any items with Mending in the player's main hand, offhand, or armor slots that are not at maximum durability
are repaired by experience orbs collected from any source.
[...]
If multiple equipped items have the enchantment and need to be repaired, one is randomly selected for each
experience orb collected. No item or location has priority for repair. 

I guess you might got confused because it only repairs one item at a time and maybe it always selected your main/off hand item.

It suddenly worked only after I equip another mending enchanted piece and then reequipped mending Diving Gear.
Maybe something is still odd but I can't reproduce it and swapping gear is sufficient workaround.