Wouldn't earthquakes and even volcanoes still be an issue? I also imagine being at the bottom of the Gulf of Mexico was of no help when the dinosaur-killing asteroid hit.

Plus where does your energy come from? You don't have solar. You need to make your own light to grow food. Geothermal is possible I guess but the ocean is terrible for corrosion and building. The high pressure at the bottom of the ocean makes everything worse.