In general aviation just because the seat is there doesn't mean you can take off with someone in it. As a rule of thumb, "one seat" is a 150 pound person, no luggage, and sometimes that's counting some of your fuel there too. A 2-seat aircraft like the Cessna 152 doesn't take off with full tanks with 2 average adult males onboard even without luggage (360 pounds full-fuel payload), and it's a struggle to get off the ground with 2 heavier adults even with reduced fuel.
The C172 is a four-seater aircraft, 577 pounds full-fuel payload (note: less than 150 lbs per seat!). So with full fuel tanks you will get one pilot, one passenger, and either some luggage/cargo or a child passenger. Maybe a second adult passenger and no luggage if nobody is too heavy (above 175 lbs average). And an over-water flight in the Pacific is not the trip to take with a reduced fuel load. So you can see how the numbers work out here: the pilot plus one obese adult might literally be the entire payload for the flight.
(Note: a 2003 study from the New Zealand Ministry of Health set their "average" male passenger weight at 187 lbs without carryon, or 207 lbs with carryon, and the average female passenger weight at 159 lbs without carryon or 169 pounds with. Further, they noted that average passenger weight was increasing at about 2 kg/6.6 lbs per decade.)
source: https://www.caa.govt.nz/pubdocs/2003_pax_weight_survey.pdf