I installed on of these. It really has been great and you get about 1000 gallons per filter. You can find much better deals from Defender and others than direct from the manufacturer. We think it is our number ONE best boat add-on of the year. Much better than filling up landfills with water bottles.
Seagull IV X-1F Purifier with Faucet - Water Purifiers and Water Micro Filters - General Ecology
You can additionally use the below water treatment method to take it an extra step. An added bonus of the Seagull water purifier is that it filters out any chlorine from the tank prior to drinking it.
How to Purify Water amount of bleach per gallon