I think this is along the line of what Triskadecamus was talking about but I’ll try and elaborate regardless.
Things do not float because they possess gas. The only thing to take into account regarding floating is how much the object weighs and how much water it displaces. This is most clearly seen with scuba tanks. An empty scuba tank (some of them anyway) can float. Fill it with air and it will sink. Why? Because air weighs something and the tank, when filled doesn’t expand in any appreciable fashion. Basically it displaces the same amount of water as it did empty but now it weighs more so it sinks.
So, when you fart you weigh less. Did the fart inside of you appreciably expand your body size to displace more water than the fart weighs? Frankly I have no idea. There are so many other variables (regular breathing, firm muscle tone vs. flabby, etc.) that’d swamp the reality out that I doubt you could even perform a reliable test of this.
My guess is you’d weigh less but still displace the same amount of water (the fart didn’t increase your body size) so you’d be more likely to float after the fart. Of course, if you just had a bean burrito the size of your head then all bets are off.