You can use a couple tools:
Allwater_wotlk, wich is a CMD tool for WOTLK ADts. It adds any kind of liquid in the whole adt at the same height. The liquid type, height and opacity (for water liquids only) are setted in the executable.
The other one is Allwater GUI, wich is a windows tool in where you can set differents liquid types and heghts on every chunk.
You can find these tools in the resource and tools forum.
Plus, read this guide:
viewtopic.php?f=78&t=3618Regards