[Sarinpa1]

whew, got me worried there that I'm not abusi- ahem, using this properly, yes yes

The amount of damage you can pull is drasticaly higher than anything of similiar required setup, yes. (Aside from cinder+knockback/pull, anyway.)
Yet to absolutely kill this sword by making it impossible to trigger a weak message would hurt diversity in builds/aproaches to combat.
To summarize,
either require a build-up of the weakness by multiple hits in a row,
or consume the water weakness upon recieving damage from first water damage source. Both those will make it compare to standard equipment/class sets, be it excell + conversion gloves/collar, cindershit, classic MA autohits or combinations of thereof.

With the weakness consumed by first attack, I'd see it as you requiring to hit with subpar basic attack, then pulling one strong water attack, leaving you with 2m. If you managed to crit with the basic attack, you've got 3m, to either do a normal attack to apply weakness again, or whatever.

I'd say that's a good solution.