Why is a previously wet light fixture now getting dimmer?
I would replace it.

However, replacing it means turning the power off to that circuit and removing the fixture completely from the wiring after using a tester/multimeter to confirm power is off at the fixture connections. That gives you an opportunity to disassemble the removed fixture as far as you can and look for corrosion and dirt. It might be possible to clean it up and remove any corrosion.