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The people of Hoenn elected Maxie as the President and he mandated that all the water be drained.
What then happened was a massive undertaking, first a massive wall was built around the region above the water. The wall was a masterpiece of human engineering, being made of Lairon/Aggron metal plates and reinforced by a thin layer melted down Registeel, it was 60 feet thick all around and 1.5 miles tall at the highest point also dotted every 100 feet along the wall is a massive pump used in the final phase of this project. When construction was complete they dropped the entire wall down into the ocean, you have to keep in mind that some parts of the wall are over a mile high so every part is a fair bit above sea level. Finally, using the power of every Plusle and Minun in the region, they powered the pumps and drained the region of all the water in and amazing 3 day pumping session. Needless the say the water level rose worldwide by about 10 feet and Alto Mare is entirely underwater but that is a small price to pay and the entire city was been evacuated before hand and now lives in the lands that were once covered with water, seaweed, and Relicanth (who are now extinct according to some studies).
It is a happy ending for all.