It’s a good question. Steel is very good at absorbing and retaining heat. Water on the other hand isn’t. Metals in general have way better thermal conductivity than water.
Exposed to the sun at the same time, a piece of metal will warm up a few times faster than the water. I believe steels can be somewhere around 8-10 times less energy required than to heat a similar mass of water.
The water could even start out slightly warmer than the bridge. Evaporative cooling would also work here since there is plenty of ventilation around the bridge. But that can slow down based upon humidity level.