Water:

When they are not on the forbidden list, you should use the following cards:
Witch of the Black Forest
Sangan
Tribe Infecting Virus
Sinister Serpent

Pot of Greed
Graceful Charity
Harpie's Feather Duster
Heavy Storm
Mystical Space Typhoon
Raigeki
Dark Hole
Monster Reborn
Premature Burial
Snatch Steal
Change of Heart
A Legendary Ocean
A Legendary Ocean
A Legendary Ocean (a third is optional)

Dust Tornado (I recommend using 5 mt removal cards)
Dust Tornado
Imperial Order
Call of the Haunted
Mirror Force
Ring of Destruction

Total: 24

WATER/Water deck supporting cards to consider:
Levia-Dragon Daedalus
Suijin
Catapult Turtle
Giga Gagagigo

Gagagigo
Mobius the Frost Monarch
The Legendary Fisherman
Mother Grizzley
Mermaid Knight
Amphibious Bugroth MK-3
Aqua Spirit
Maiden of the Aqua
Star Boy

Tornado Wall (2-3 if you have 3 or more more cards that can count as UMI)

As a set:
Torpedo Fish
Orca Mega-Fortress of Darkness
Cannonball Spear Shellfish

Other (non-water) cards to certainly consider:
Breaker the Magical Warrior
Exiled Force
Magical Scientist
Cyber Jar
Fiber Jar

Fissure
Fissure
Nobleman of Crossout
Nobleman of Crossout

*Torrential Tribute
Magic Cylinder
Magic Drain (to help protect your Legendary Ocean and Tornado Wall)




Advanced Format (October 1, 2004 - April 1, 2005)
 
Traditional Format:

Levia-Dragon Daedalus
Mobius the Frost Monarch
Giga Gagagigo
Giga Gagagigo

Mother Grizzley
Mermaid Knight
Amphibious Bugroth MK-3
Aqua Spirit
Maiden of the Aqua
Breaker the Magical Warrior
Exiled Force
Cyber Jar
Fiber Jar
Witch of the Black Forest
Sangan
Tribe Infecting Virus
Sinister Serpent

Pot of Greed
Graceful Charity
Harpie's Feather Duster
Heavy Storm
Mystical Space Typhoon
Raigeki
Dark Hole
Monster Reborn
Premature Burial
Snatch Steal
Change of Heart
A Legendary Ocean
A Legendary Ocean
A Legendary Ocean
Nobleman of Crossout

Dust Tornado
Dust Tornado
Imperial Order
Call of the Haunted
Mirror Force
Ring of Destruction
Torrential Tribute
Tornado Wall
Tornado Wall