After 10's of charters all over the world. In the BVI do not worry about it. Do not waste any emotion or drama over water. There are plenty of places to get water. Start by putting an extra gallon of jug water on the boat for each person if you should happen to run out. Until you figure out your crews consumption habits and preferences. Get water whenever you can. Make sure everyone gets a turn holding the hose. I find once you hold the hose filling the tanks. Everyone gains a better respect for water. The same goes once you have to clean up sand by the way. Use of the aft shower and buddy rinse approach can also be a big help on the water frugality efforts. The key whether you are rinsing dishes or bathing is to never leave the water running. Turn it on, turn it off the open running tap is the threat or the enemy. Note: I said do not waste your energy worrying about this in the BVI. But, with today younger set. I am not sure the measure of gallons or liters means anything to them. Most do get minutes. Take a bucket or jug and time how long it takes to fill one gallon. Divide by your capacity and determine the estimated number of minutes of water the boat has. That make the amount of water relative to a wider group. Then there is that reminder that someone must hold the hose to put every gallon back in. Have fun! All the teenagers who have traveled with us have without issue.