The A/C cord on a shorepower system is a 30 AMP or 50 AMP twist lock cord, and has much heavier insulation and weatherproof connectors.
If you're going to want to install a full shore power system with an inverter
/charger, you're looking at some serious weight on-board, as you generally need a pretty hefty battery bank to use an inverter
You will also want to use marine GFCI outlets for the A/C outlets on the boat...to help prevent anyone from electrocuted.
Have you taken a look at the electrical load of the things you want to use on your boat? How many watts do they draw in use, and how many hours are you planning on using them. Also, have you done this for the boat's DC side—electronics, lights
, instruments, etc.
Generally, your battery bank should be 2-4 times the total average electrical load that it will be used for between recharging. Also, how will you plan to recharge the batteries?
A couple of other caveats—I wouldn't recommend using a non-marinized stereo on a boat as small as a Cal 20, as the smaller boats tend to be fairly wet, and the salt water/salt air will quickly attack and corrode a standard home stereo system. Also, a standard home stereo doesn't really tolerate being thrown from side to side, as will happen on a small monohull sailboat, and will need to be anchored down some how. It also may take up more space than a similar-capability marine stereo system.
I hope this helps a bit. You really need to do a serious amount of planning, otherwise you'll end up throwing your money away when doing a project like this.