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.
Also acts as the Field Spell Umi on the Field, without the ATK and DEF boosts.
If there are other Field Spell on the Field, the monster’s effect is not applied.
Weak ATK stats susceptible to position switching plays.
If Maiden of the Aqua is your only source of Umi presence (i.e. you do not have any other “Umi” Field Spells active), you can still banish Maiden of the Aqua via Sea Stealth Attack’s effect, but your Sea Stealth Attack will not be active without the presence of Maiden of the Aqua for the remainder of that turn. Maiden of the Aqua will return to your Field during the End Phase it was banished.
Normally players would try to counter Sea Stealth Attack by eliminating the Field Spell Umi / Legendary Ocean. This monster will act as Umi to allow Sea Stealth Attack to continue to function, and cannot be eliminated easily via traditional ways like Cosmic Cyclone.
Together with Fishborg Planter, you can tribute Maiden of the Aqua to Special Summon Citadel Whale to the Field. Citadel Whale in turn searches for Sea Stealth Attack and sets it on your backrow. Use Powerful Rebirth at anytime to return Maiden of the Aqua to your Field in Defense position, in order for you to activate Sea Stealth Attack.