Plumbing a boat can be a daunting prospect. When taken in bite sized chunks the task is a lot easier. There appears to be a myriad of different fittings fired at the marine, caravan and motorhome markets that look nothing like their domestic equivalents.
In practice fortunately things are much simpler.
It is possible to interface domestic fittings with boat systems, sometimes the only area of creativity will come when attaching domestic waste pipe to flexible hose to connect to through hull barbed outlets.
More creativity is required for shower waste outlets as they exit,by their nature, below the waterline. The motorhome market is a good source of low profile products here, many of which are finding their way into the traditional chandleries
as boat owners tastes change.
Your local plumbers merchant can be an invaluable source of wisdom if you have a problem that needs an unusual fitting for a solution.