Is any of Earth’s ocean water extraterrestrial? The asteroid belt is reportedly the remains of the water world, “Tiamat.” In the age of the dinosaurs, Earth was generally considered one big swamp devoid of voluminous oceans. Did Earth inherit any of its water from the destroyed Tiamat?

This again is a mixing of ideas. The Earth at one time had far less water available and this was imported to provide a water-rich environment and it did come from that body named Tiamat and was a divine manifestation. So the water was transferred and repurposed to serve the Earth following a cataclysmic event befalling the original planetary body.