Strong Water Runes need a Oceanstone along with a Blank Rune; a Silver Ingot and three Normal Water Runes to make one Strong Water Rune.
Strong Runes can be crafted at level twenty at the Rune Altar.
You need three of these to learn Tidal Wave and you will need extra to cast the spell.
The command to make a Strong Water Rune is [nya!rpg rune altar strong water]. For more info use [nya!info strong water rune].