If I have Maiden Of The Aqua on the field whose effect says that While face up the field zone is treated as umi, but there is no increase of attack and defence. With Levia Dragon On the field can I activate his effect (destroy umi to destroy all cards on the field except this one

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No...simply put...

As long as this card remains face-up on the field, the field is treated as "Umi" (however there is no increasing or decreasing of ATK/DEF due to "Umi"'s effect). If there is an active Field Spell Card on the field, this effect is not applied.

The NAME itself is not treated as "Umi" it only makes the field as IF Umi were there, except for ATK/DEF increase/decreases (so stuff like The Legendary Fisherman and such will work).

However, Levia-Dragon - Daedalus CANNOT work as there is nothing with the name "Umi" to send from your field to the Graveyard. Just "Maiden of the Aqua".

[Re: Levia-Dragon - Daedalus] You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon Daedalus"' effect, but you can't send "Maiden of the Aqua".

Hope that helped.

Also, for further proof, here's the UDE/Netrep rule

[Re: Levia-Dragon - Daedalus] You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon Daedalus"' effect, but you can't send "Maiden of the Aqua".

Maiden of Aqua is not Umi. She treats the field as Umi. Obviously you cannot send the field to the Graveyard.

A Legendary Ocean IS Umi.

Umi is Umi.

Maiden of Aqua cannot be sent since she only makes the field change, and is in no way part of the requirement for Levia Dragon's effect.

Send the field to the Graveyard, lol...*crumples up the current mat, takes out another and puts the crumpled one into the Graveyard*...you're right, it doesn't fit...XD