Open Brainstorms

Never clean it and perhaps set up some fake green fungus-like stuff in the corners. Then always only have almost emty durty bottles standing around, with suspicious looking things in it. And off course a shower curtain almost covered with black stuff..

Use a randon timer set for times less than what you spend in the shower now to turn the water cold

Find out if it's b/c they are groggy in the morning; if so, make them coffee... they'll love the thoughtfulness! And you will have solved the problem without a confrontation.

Teach them to be sensitive to other people's needs. This way, if their showers bother you, they will try to accomodate you.

Lobby the government to creat a bill that would tax people who use too much water.

What about enforcing a policy that you can only take 3 showers per week instead of every day.

Discuss this problem with other family members. I am sure they would understand your point and co operate

What about using a low-flow shower head?

throw cold water on them or cut off the water supply after 3 minutes

praise them when they spend less time than usual.

Following on from #1, the shower water could become cooler as time progresses. Your family may hate you for that though!

What I would like from a shower is a brush that attaches to the shower head. Would both save a lot of time and provide a more hygenic wash. I've googled and there are some in existence but what would make this product succesful is a universal brush that can attach to any shower head.

Put a timer / clock in the shower