No, not all C Dory 25 come with holding tanks. I have head of them with porti pottys, composting toilets, and the self contained, Sea Land Traveler, with has a 9 gallon holding tank which is built into the base. My 25 (2003 vintage) had the holding tank aft, and it was a pain. If I had kept the boat, I would have put in the Sea land Traveler, with the base holding 9 gallon tank. All holding tanks can be pumped out with a vacuum system, or with a macerator pump which pumps overboard. There are several types of overboard discharge pumps, from the tanks, including centrifical grinding macerators, and diaphragm pumps like Whale.
It is illegal to discharge any sewage into waters of the US, except at sea--and the distance varies, but generally 3 miles. (I know some cities dump right into the seas right in front of their beaches!)
However, it will be most unusual to have a direct discharge boat in the lower 48 states. It can be done with a Y valve, but there are areas where you may run afoul of various law enforcement agencies.
Here is a PDF document about many different types of heads. The Sea Land is included toward the end. Dometic is buying up many other companies, and the head industry is no exception.
http://www.dometic.com/QBankFiles3/...e/2911-Sanitation-Product-Guide-Web_14750.pdf