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