Bought a used boat last fall,salt water flats boat, & it has a canister in the fuel line that I was told was a water separator. Read on another forum that gas w/ethanol can attract water & go bad alot quicker. How often should I change this filter or clean it & is it necessary in fresh water?

Copied the following from another forum: 

There is lots of discussion about this in the boating forums. If you put it in and run it right away, you won’t have any problems. The problems occur when you have to leave it in your tank. Alcohol in gas is hygroscopic - it attracts and blends with water. As your tank breathes in and out from heating and cooling each day, water (humidity) goes in but gets attached to the alcohol molecules in the gas and stays. After a point, it gets saturated and you get “phase separation”. The water/alcohol mixture falls to the bottom in large quantities and when you suck that through your carbs BAD things start to happen. A water filter will help, but unless changed very frequently, will quickly be overcome with the shear volume of crap in your tank.

There are two ways to cope:

1. Keep the tank absolutely full. A full tank has no room to breathe therefore less chance of water being absorbed. The downside of that is that if you don’t use it enough the gas will go bad. Ethanol blended gas goes bad in as little as 60 days.

2. Keep the tank absolutely empty. After each trip, totally empty the tank and use the gas elsewhere (I run the pre-mix in my lawn tractor - smokes a little but no other bad effects). Before the next trip, add about a half gallon of gas to the tank and manually pump it through the filter. Empty the water separator before and after doing this. Then add as much gas as you think you’ll safely need for that trip. Empty it out when you are done.

I’m doing the latter method. I disconnect the hose from the motor and drop it over the back of the transom into a gas can and use the bulb to pump it out. Usually, I can start a good siphon and it will run on its own.

Depending on what part of the country you’re in, non-ethanol gas may be available.  There are several stations nearby here in MS that carry it because of all the agriculture.  It is common knowledge around here that ethanol in gas ruins fuel pumps.  Many older tractors, generators, 4 wheelers etc. will not tolerate it.  I had to replace a fuel pump in a Polaris 700 EFI with only 42 hours on it because I didn’t know this at the time.  I’ve got a 25’ center console and only use non-ethanol in the Yamaha 250 outboard.  I wish someone would explain this to the legislators in D.C.  But many of them have their own problems with ethanol…..and they want to run our health care.  Good luck, and God save us all!!

I change my filter every year. If you run your motor a lot you can’t go wrong changing it more often.
I don’t have the non ethonal around here so I use startron in my tank with every fill up