It should work like this:
A mod called MLG Water Bucket can already do this. But there's some problem:
1. It's a Forge mod not Fabric.
2. Only 1.16.4 or 1.16.5 is supported.
3. It has a reported issue: It doesn't MLG at lower heights like 7 blocks. (It can be changed manually by editing it's Config file. But who does that?!😪)
It should follow:
1. If the player has a water bucket in hand, they will look down and right click right before landing to avoid damage then immediately pick up the water before it spreads.
2. If the bucket is in the hotbar, but not the main or offhand, the hotbar slot will switch to the bucket's position.
3. If the bucket is in the inventory, but not in the hotbar, it will be transferred to the hotbar, then to the main hand.
Vape V4 a hacked client already has something like this.
MLG Water Bucket's source code will surely make the progress faster of making this mod.