Do I really need a water fuel separator?

I'm a complete novice with outboards and just acquired this 2005 6hp 4 stroke. While trying to educate myself on the do's and don'ts I came across someone saying that every engine should have a water fuel separator. Is this necessary for even the smaller sized engines?

When I bought the boat (12 foot aluminum) and engine it came with a portable gas tank already so I'm not sure how old it is. The tank has some cap on it ever so slightly open (which I think you're supposed to have open while operating) and was sitting outside for a month at least so I'm concerned about water in the tank.

Does it make sense to get a water fuel separator or should I just change the gas and call it a day? Any other suggestions or comments to a novice are welcomed.

Thanks all.

Knowing the exact model will always help us. Year is unimportant. Assuming you have a Tohatsu MFS6A2. You can DL your owners manual from http://www.tohatsu.com/tech_info/manuals.html
A water separator will reduce the possibility that water gets ingested with your fuel, but is likely impractical on your motor, unless you have a convenient place to mount one. I run a separator on my 9.8 on the sailboat, but my 6 has the integral tank, so there isn't any provision for a separator on my dinghy.
Yes, you want the tank vent open while running to allow air to replace the fuel that gets consumed. Close it when not in use. Keep the tank out of direct sunlight as much as reasonable.
If the vent has been open for a month in the rain, it's recommended to dump the gas.
In the first place, US gas is already spoiling after a month. Only get as much gas as you can use in a couple of weeks, and try to buy non-ethanol gas.
Likewise, if it's E-10 gas, it is possibly already getting to the saturation point where the accumulated water vapor, combined with alcohol, drops out. That's called phase separation, and the resultant stuff in the tank is pretty unusable.
Always disconnect the fuel connector at the motor at the end of the day, and allow the carb to run dry to avoid both carb varnishing and over-pressurization of the fuel system as the sun heat/cool cycles cause expansion of the gas in the tank and primer.