Not sure what engine you have, however Volvo Penta normally puts the water in fuel sensor on the secondary fuel filter that is on the engine. The sensor is at the bottom of that filter. If it is like the D3 150 water in fuel sensor it is easy to remove. There is a connector attached to the sensor that can be removed by pushing in on the metal clip and pulling the connector off the sensor. The sensor can then be unscrewed by hand, it should not be tight. Make sure you have a container under the sensor to collect the fuel. A pint or quart container should be sufficient. Then you can look for water in the fuel it should be at the bottom of the container. I would also drain the external fuel filter which should have a plug at the bottom.
Be careful when you screw the water in fuel sensor back into the secondary fuel filter, it is plastic and can be cracked If overly tightened. My instructions said to screw it in until the rubber washer just touches the filter and then screw it in another 1/2 turn.
If you do find water you can either let the filters take it out over time by periodically draining the filters or have your tank cleaned. If you find other nasty looking stuff in the fuel (the fuel should be a bright pink color) that is drained I would definitively have the tank cleaned. Your marina should be able to help with tank cleaning.
Good luck.
