Waste is always a tricky topic for boaters, because it's something you've got to deal with yourself. Keeping surrounding waters clean and free of contaminants is important. The primary environmental concern with sewage is not the urine, which is sterile, but feces which contain bacteria, pathogens, and nutrients, and should not go overboard. The septic tank may only be emptied, from at least, 3 miles from shore.
Pump-out stations will help you get rid of waste, but are only available near a handful of cities. Composting toilets might be a good option for those who do not want to haul anchor and head offshore to flush their tank. Having a composting head aboard frees up much-needed space. It also means no holding tank to empty, no smelly hoses to unclog, and little to no maintenance. You can build your own composting head or buy one of the many models on the market.
Most models have a urine diverter, separating the liquids from the solids to ensure contamination-free composting. Some models also have fans, to help the solids dry out and remove odors. In composting heads, urine accounts for ~85% of the waste volume in tanks. Having a diverter is great because it increases capacity for solids, which means not having to empty the tank as often. The ideal medium for processing your waste into compost depends on how you plan on using your toilet and vary between sawdust or peat moss. C-head wrote a very good article on the subject.
Disposing of the waste requires a bit of planning. If near a city, partnering with someone on land with composting facilities is ideal, but otherwise, bringing the solids to the trash in composting bags is the next best thing. When full, it's possible to empty jugs of urine into public toilets. If there are no facilities nearby, capping it off to dispose of later, and replacing the jug is a good option.