We use our Catalina 309 for daysailing and the occasional weekend or 2 week trip. We took out the smelly, troublesome head that came with the boat and put in a porta potti. For the way we use our boat, the porta is superior to a head and holding tank in just about every way.
It's important that you use the chemicals in it, and empty it at least every two weeks. Smell is not an issue. Less than with the stock system.
Emptying is very easy with our Thetford brand porta. Just dump the lower portion of the porta into a toilet. There's a large pipe that swings out for emptying, making it clean and easy.
Anyone using the stock toilet on a 309 (a typical marine toilet), is going to be very sad when they or a guest takes a substantial dump. Hey, captain, have fun cleaning that up. The porta will take anything you throw at it.
Worst case scenario is that you have to dump it overboard when nobody's around. I hear fish bypass the porta and poop directly into the environment. Imagine that