The topic of what frequency to monitor has been discussed and is most relevant to the Salty Dogs forum. It is generally agreed that all VHF users should monitor Ch. 16. Beyond that there are several channels various people monitor; 11, 66, 68, 69, 77 and 78 are most frequently mentioned. Because all the channels are public, others are also using these channels so cell phones are also popular if conditions permit.
The VHF rules say that if you have a radio on, you are required to monitor 16 unless you are using it for a call. You must provide assistance if you are within range of a vessel in distress provided you can do it without endangering your vessel or crew. Those distress calls should be originated on channel 16. If you only monitor 68 or some other channel, you may miss a distress call where you could be of assistance.
Once you contact someone on 16 (or 9 which is the other hailing frequency for recreational boaters), you immediately select another channel to converse on. After you are through, you go back to 16.
My radio is set for priority scan. It monitors 16 and 9 and then the public channels where recreational boaters are allowed to talk.
If you aren't familiar with the correct way to use your radio, the FCC has the rules for operating a VHF here:
Put a CB in your boat. $40 at Kmart for a tiny Midland,,,then you can talk to your buddies and eavesdrop on the other guy. Listen to 13 & 33 in Astoria. Most all the local guys are on the CB, and they use their VHFs for monitoring and communication with other vessels and the Coast Guard. The Fcc asks that you not tie up a frequency for more than two minutes on a VHF, so CBs are the answer