Ive got a mercury 150 optimax 2005 and ive got a problem with water getting into my petrol tank I was told to drain off the tank and start again,so I drained 78 gallons into clear containers with no sign of water in it.so it all went back in.changed the filter and went fishing, after a bit of running, off went the beeper four times to tell me the problem was back.I had to remove the filter and clean it out five miles offshore which was quite tricky hanging over the stern.now what I want to know is can I put another fuel filter inline fixed into the boat well before it gets to the engine so i can monitor it and empty easier.I must also add I cannot get to the 120 gallon tank as it is sealed below deck could it be condensation? very grateful for any advive