Nothing beats the work of enzymes to keep drains clear both in your boat and at home. Natures Miracle found at pet stores contains enzymes that ‘eat’ food particles and sludge in drains. Pour a cup down the drains at night and let them "eat" away at all that buildup. It works at home as well.
Eliminate soap buildup in showers by switching soap. Use a liquid or glycerine soap to shower and wash your hands. The talc in bar soap builds up on the sides of drains eventually clogging and congesting them so they don't drain properly. Switch soap and solve that problem.