I did the math. When we're cruising, we use approx 8 gallons per person, per day for the two of us.
That's all in. Drinking, coffee, cooking, shower everyday, washing, etc. It also include one load of laundry per week. I've even been known to spray the teak cockpit floor down with with the transom shower occasionally, est 1pt usage. No salt water usage, but typical conservation practices. Pretty decadent I suppose, but our water supply lasts 14 days for the two of us. That's how I got to the above per day usage, which includes some bottled drinking water of about 0.5 gallon per day per person. The biggest bottled water usage is for coffee. Needless to say, we could dramatically reduce this usage by skipping showers, using salt water, not running laundry. Just don't need to.
Personally, I want to be able to be away for a minimum of 10 days without needing to land for water. Whether I plan to or not, its pretty unlikely we would be off a dock somewhere for longer, so this consumption works and no one feels like they are in prison.
I also don't think there has ever been an exception to having someone along for a day here or there, while we're cruising. Obviously, this increases consumption, but we top off when we pick them up and again when we drop them off. Simple and never a concern over usage, as a result. We just had another couple aboard for several days last week that were new to boating. Easy, as they were accustom to water conservation when camping.
The number one trick for conservation isn't the obvious on/off, don't let water run when you don't need it approach. It's to get everyone in the habit of not turning the water all the way on at all. You never need to, unless filling a pot. Think about the slug of water one runs over a toothbrush at home. I bet you use a pint of water to brush your teeth. Now consider the trickle it would really take to wet the brush as much as that slug does.
The best way to conserve water when showering is the dish sprayer head, usually found on transom showers. On when you need it and off the second you don't. My wife will use our transom shower most often when cruising in warm weather. I would love to measure the difference from the head faucet down below. Just the time it takes to reach over with soap in my eyes to find the lever, after I don't need it on anymore, must really add up over a week or two.
I suspect our usage level will suggest to some that we hardly conserve at all. However, we do. We just don't deny ourselves the ability to use water for whatever we need. If we used our supply, like we were at home, it would last exactly 3 days.
Finally, we hold water in two equal tanks, mounted in the bilge on each side of the keel. While a good location for the weight, we still run one down to half, switch to the other and run it empty, then switch back to the first. Both weight management and the best way to always feel you have a backup. It also keeps the supply circulating with new water, if you're not using it up on a longer cruise. Admittedly, most of ours are only a few days away from our slip at a time.