That’s a good question – tricky – I don’t think there is an accepted answer to this one – one theory that I’m familiar with is water-rich meteorites/asteroids bombarded the earth bringing water with them.
Hydrogen is the most abundant element in the universe and oxygen although far less abundant is very reactive so it’s not too surprising that icy asteroids formed and then that they bombarded the earth….
Oceans are amazing – they cover 70% of the Earth’s surface but contain 97% of all of the water on Earth!
The oceans and seas are the source of water vapour into the atmosphere to produce rain for freshwater, which eventually makes its way back into the sea. Not only that but the oceans also affect weather and climate.
The oceans also amazingly contain 50-80% of all life on Earth – absolutely amazing!
What an interesting question. I suppose there is sea, because such a large proportion of our planet (at least on the surface) is made up of water. But that just leads us to another question, why is there so much water?