The ability to search for “A Legendary Ocean” Field spell is a great advantage as it instantly lowers all WATER type monsters level by 1, that means you can instantly Normal Summon Legendary Fisherman II, and use 1 Tribute instead of 2 to Tribute Summon Citadel Whale or Levia-Dragon - Daedalus.
Once you have discarded Warrior of Atlantis, you can Special Summon it with Powerful Rebirth. Sea Stealth Attack allows you to banish Warrior of Atlantis, you can protect all your Spell/Traps for that turn, and your Warrior of Atlantis returns to your Field at your End Phase.
"You can choose to use Lemuria, The Forgotten City as well to increase
its level to over level 5 in order to use Sea Stealth Attack’s effect.". No. Stealth says "if your WATER monster whose ORIGINAL Level is 5 or higher".