The problem is that that's a complete waste of 2 refresh if you're playing at submerged with evocation spin rules. You can easily get a free weapon 3 attack, and if you feel like trying to exploit the rule, you pretty easily get a weapons 5 attack that costs no mental stress.
Indeed, I've been grappling with this for a while (the temp magic weapon thread) and I had the same problem. In the same way, you could use an enchanted item slot and use it like you might a potion but, yet again, why should you have to when a submerged character should be able to do it with standard evocation.
Unfortunately, I don't think there is an easy answer. The most obvious, common sense idea would be just to cast it as a spirit evocation and add +2 to the damage... but then every wizard will end up carrying throwing knives, bolas and boomerangs around.
Hey, now a bolas would be awesome! Damage plus block manoeuvre wrapped up in a single sexy package