Unless you're in a resort or a public restroom, the water that feeds the toilet probably comes from rainwater. If you use all that water up then people won't be able to take showers. Take your showers by first wetting your body, then turning off the water and lathering up. Turn it back on to rinse. There's also a saying regarding the toilet -- "If its yellow like the sun, let it shine -- if its brown, flush it down".
One of the nicest customs -and virtually mandatory if you want to get an answer from an islander - in the Virgin Islands is to initially start any and all conversations with "Good (Morning, Afternoon, Evening), how are you?" and then give them a moment to respond. THEN you can ask a question. This is common even in the grocery stores, buying ferry tickets, catching a taxi, getting on the public bus - do it every time and your time in the islands will be happy, mon!