It depends on several factors.

The condensed version is that every kind of supernatural ability the user has that [i]isn't[/i] based on the element provides interfeerence.

Starting from an ordinary human wielding Aqua as "0":

A user with only water based abilities - 2
A user with mixed water and other type abilities - 1
A user with no abilities - 0
A user with only other type abilities - -1
A user with fire based abilities - -2

At -2, the sword begins rebelling against the wielder. Someone with fire based powers could technically wield Aqua, but the sword's magic would fight them every step, and they'd probably become crippled.