A Monster you Don't Want to Face
The cause of the explosion being the Edenian submarines that were lurking under the fleets, following them since when they invaded Esparias water.
They were nothing but state-of-the-art in everything when compared to the latest gen of any country's military.
Starting from the energy, these submarines were each being powered by harnessing the energy of a star in the form of fusion reactors. This means that they won't need to ever be refueled as long as they are under the surface of the water. Because these reactors would efficiently extract deuterium, a fuel source, from the surrounding water, rendering themselves self-sustaining during underwater operations.
As for the propulsion system it too was the state of the art magnetic propulsion helping the submarine in its stealthiness while at the same time allowing it to go at unimaginable high speeds.