Well it wasn't a "sea", it was a vessel. So the question is simple. Assuming we know what a cubit measures and assuming the diameter of the vessel, not the rim but the vessel, was 10 cubits how many cubits should the guy with the cubit measuring line have measured?
Oy vey!
If the diameter is 10 cubits, and if pi were the same then as it is now, it should have measured 31.41579 [etc.] cubits around. Does that answer your question?