Bonding

Hydrogeddon, Oxygeddon, and Duoterion are the three main deck monsters the archetype depends on, each present to feed the Bonding spell and trap cards that ultimately bring out Water Dragon or Water Dragon Cluster. Hydrogeddon can replace itself when it destroys an opponent's monster by battle, gradually stacking the field with bodies ready to be tributed. Duoterion does the heaviest lifting in setting up plays: discarding it searches any Bonding Spell or Trap from the deck, and when it is summoned it can revive a Hydrogeddon, Oxygeddon, or second Duoterion from the graveyard, giving the deck a degree of recovery that compensates for how tribute-hungry its summoning conditions are.

The payoff for assembling these pieces is substantial. Water Dragon reduces every FIRE and Pyro monster on the field to zero ATK, and if it is destroyed it returns the tributed Hydrogeddon and Oxygeddon from the graveyard so the engine can restart. Water Dragon Cluster goes further still, blanking the effects and ATK of all opponent's Effect Monsters the turn it arrives, and can then tribute itself to bring out two copies of Water Dragon directly from the deck. Bonding - D2O and Bonding - DHO widen the tribute pool by accepting Duoterion in place of Hydrogeddon, with D2O recycling itself from the graveyard whenever a Water Dragon leaves the field.

Carboneddon sits outside the main H2O engine, offering an alternative route by banishing itself to special summon a Level 7 or lower Dragon Normal Monster from the hand or deck, which can bridge into extra deck plays and give the archetype a second angle of attack when the tribute pieces are not yet assembled.