What was the bronze Sea in Solomon's temple used for?

Answer: Washing by the priests

The ten basins were for rinsing burnt offerings, but the Sea was for the priests to wash in.

“He also made ten basins for washing and placed five on the south side and five on the north. The parts of the burnt offering were rinsed in them, but the priests used the Sea for washing.”— 2 Chronicles 4:6 (BSB)