Slight bit of overpower I've found through practice. Waterbenders can deal 7d4 damage at first level, half of which is not subject to DR. A first level fairly optimized waterbender can get a bending check of 21 reliably. 10+4(mod)+(3 skill focus)+4(ranks). Using the ice shards seed, that's 10+10(extra shards)=20. Easy, for 5 extra shards. total, 7. Thrown in a 30-foot cone, reflex half. At second level, you can deal 18d4. Half cold. Best of all, you can split it up however you like among the targets in your cone.
The main problem here is the ice shards seed itself. While made of awesome(masterwork weapons? MADE OF ICE! I rest my case.) , I recommend 3 points of check/shard instead of two.