You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon Daedalus"' effect, but you can't send "Maiden of the Aqua".
You can't send a face-down "Umi" or "A Legendary Ocean" to activate "Levia-Dragon Daedalus"' effect.
If you activate "Levia-Dragon Daedalus"' effect and your opponent chains "Ring of Destruction" to destroy "Levia-Dragon Daedalus", "Levia-Dragon Daedalus"' effect still resolves even though it has been destroyed.
If an effect is chained to “Levia-Dragon Daedalus'” effect to flip “Levia-Dragon Daedalus" face-down, it is still not destroyed by its effect.