If your water magic skill is 30 each shard should deal 72 points of damage. That's what they do in my game (unless the monsters resist part of the damage).

However, the shards don't "ricochet until they hit something or melt" as the manual describes (they do in mm6 iirc?). And if you shoot it at a monster that monster won't take any damage at all; the shards will just expand in a circle from it.

It's just a bad spell since you have no control of it. Even if you succesfully fire it into the centre of a group of monsters the shards may still miss. A monster will at most be hit by a single shard, that's why the spell is so awful.

Death Blossom is somewhat similar; even with a good aim it's bad.