Channel 16 for hailing and emergency ONLY. Call a boat, get a response, then switch to another frequency.
Most fishermen in our area monitor 65 or 68.
For bridges, channel 13. Good to hear any traffic from boaters coming from the other direction, when the bridge will re-open, etc.
Channel 9 for radio checks (or any other channel, except for 16).
I ALWAYS have my radio on. I monitor channel 16 and a second channel, usually 10 during the summer hours (other boats from my marina use that channel, less busy than 65 or 68). In the spring (before the 'weekend' boaters get on the water) I will monitor channel 65 or 68.
By listening to channel 16 last night I was able to get off the water before a fog bank rolled in after I heard the CG issue a safety bulletin about the fog on 16.