VHF is really only
line of sight communication, although there are times when you can hear and reach out much further. Your policy of monitoring it at all times is commendable. It is not so much that you may need it, but that you may be the only one who hears that important call from another vessel in distress. Never, ever, think that everyone else heard what you heard. With experience you'll find that other vessel's and even the CG will respond with, "what distress call"? It's just due to the vagaries of
VHF propagation. And being ready to aid a distressed vessel is the beginning of true seamanship. We operate in an environment where we are all bound together by the laws and traditions of the sea. Complete strangers will come to our aid. There is no order of contempt strong enough for those who do not respect those traditions.
Instruction for the novice using it in an emergency is simple; make sure it's on Channel 16 and release the transmit button after speaking. Everything else is detail for safer times.