For what it's worth...
Five of us took the boat to the Dry Tortugas (what a fitting name!) for five days of diving.
Island Breeze carries just under 400 gallons in two tanks. After five days with at least one dive a day, and two on several days, we arrived back in Key West operating on the larger tank, which holds 187 gallons. Bear in mind that we picked the wrong time of year for the trip, and had to deal with 'Sea Lice' (baby jellyfish), which meant a good rinse after each dive. Diver and his equipment, too or misery followed.
Meanwhile we cooked, cleaned, etc. No, we didn't waste water, but we didn't scrimp, either.
BTW, we do have a watermaker aboard, capable of making 400 gallons a day or 16.7 gallons an hour, but it has never been turned on since it was installed. The 400 gallons have taken us to the Bahamas, the Dry Tortugas and I can't tell you how many dive trips in the keys, and we've never used more than one tank of water unless there were women aboard. Even then, the second tank was barely touched.
That all being said, the delivery crew (4 guys) who brought the boat from South Africa to Ft. Lauderdale ran completely out of water before they hit Antigua, and that run took them 5 days. The delivery captain left his own notebook and the official ship's log aboard, and that bunch were, IMHO, lucky to survive the trip. They ran out of water a day and a half out of Antigua, ran the genset without oil and seized it, got it running again and the blown out front and rear seals blew oil all over the engine room. They nearly had a mutiny because one of the crew was...odd.
For myself, I like to think that you can get by on about 2.5 gallons of water a day unless it is blistering hot, which the run from LA to Cabo will be. With that in mind, either carry more water, or an awful lot of gatorade. Becoming dehydrated leads to heat stroke, and that is no laughing matter when you're offshore. I had to start an IV on a guy in the Bahamas to rehydrate him, and it was still nip-and-tuck for a few hours.
Rule of thumb: If your urine is anything but faintly yellow, you're dehydrated. If it's dark yellow and stinks, you need to stay out of the sun, and drink as much NON-ALCOHOLIC liquid as you can, and then drink some more.
Have fun, and safe voyage.
S/V Island Breeze