Aqua Tail, 90 power, 90 accuracy
Waterfall 80 power, 100 accuracy ( 20% flinch but both Pokemon are to slow for it to matter)
both physical attacks
Aqua tail is tempting, but the 10% chance of a miss happens a lot more often then you would think.
Waterfall is better. However Blastoise is really more of a special attacker...