If you were using 270 gallons of water per year for 3 people just to brush teeth then you do need to work on conservation. If I am doing the math right that works out to about a quart per day per person.
Why so much? Do you leave the tap running while you brush? I could brush my teeth using a half cup or less of fresh water. Instead of making a special, waterless tooth paste why not learn to use less water in the process.
Other suggestions are all pretty standard and mostly already mentioned.
1. Salt water shower
with a final rinse of freshwater. Once clean and rinsed with salt water
pour just a cup of fresh on your head
once or twice, rinse it well through your hair and let it run down your body taking the salt
residue away. You should be clean and salt free and no sticky feeling on your skin.
2. Same with dishes.
3. Steam vegetables instead of boiling (it's also healthier). Be careful cooking
with salt water as you may end up with over salted food
in some recipes
Other than these I can't think of other places I use fresh water on the boat so no other major savings.