Notes & Combos
This deck has legs but I feel like it needs work. ED is basically every Water Synchro worth playing generically except Packbit and old Brio because I didn't want to craft them. Probably throwing in a Heavy Infantry would help, but I didn't have it in here so I'm not submitting it with them.
The gameplan is pretty straightforward. Lots of ways to get to your full combo without a normal, combined with a solid normal in Diva. If you can get to Neptabyss without Diva, Diva summons Duonigis, Duonigis doubles Diva's level, then Diva makes Prima Donna and searches whatever you're missing, usually Aegirine but sometimes Neptabyss if you know what they have. End board should include at least Gymir, but also often includes one of or both of Chengying and Dragite.
ED monster notes:
Repetiteur is in here as a search target with Synchro Overtake. It is mostly useless but sometimes you can use it to dump a goon.
Prima Donna makes Chengying or Aegirine untargetable from monsters, which makes them significantly harder to remove and I have yet to run into a deck which has been able to get over a Chengying that stuck.
Tatsunoko can clear up hands if goons got Called, or it can convert an Aegirine into Coral Dragon. There's other stuff it could do possibly but those are corner cases not worth talking about.
White Aura Whale is just a board wipe if your opponent doesn't summon in defense.
Brionac, Coral Dragon and Archethys are different flavors of removal.
I am sure Navy Dragon Mech has a purpose, but I haven't found it yet.
Crystron Quandax could probably do something in here, but whatever it is does not include summoning White Aura Whale or either of the Trishulas, and I don't have a way of making a 4 in this deck. If making a 4 does become doable, though, it suddenly replaces either Navy Dragon Mech or one of the Chengyings or Gymirs.