That was not merely a sea monster. That was the demon god Osial, intending to destroy Liyue with storms and floods.

[ And as for the dragon... what a strange thing to ask the man who was that dragon. It's hard to envision why she would not know of Rex Lapis being a dragon, but he'll humour her once again. ]

As for the dragon... that is Rex Lapis, the Geo Archon.

[ Almost everyone in Liyue knows how the Geo Archon looks like. He wouldn't mention that to Rose outright, though. ]