Mike, you have a simple question, but the answer is complicated.
SeaRey fuel tanks have had various capacities. The original, rectangular fiberglass tank held 18 gallons, with about 3 gallons unusable when descending. Later "plastic" tanks have held up to 28 gallons. Mine--and my favorite of all--is aluminum, made in Australia: 23 gallons, with zero unusable. That gives my Rotax 914 more than 4 hours of cruise, plus more than half an hour reserve. I'm almost always ready to come down in 3 hours anyway.
It's possible to make the engine burn more than 5 gph; but that won't make a SeaRey go much faster. You can also cruise at 4 gph if you're not in a hurry to get somewhere. (We do these calculations every time we fly ANY aircraft, by the way. Most of the time it's a balance of passenger weight v. fuel.)
The useful load depends on how the SeaRey was built and how it's registered. Empty weights have ranged from less than 900 lbs to more than 1100 lbs. 1,000 is more or less typical, these days.
But the maximum takeoff weight (gross weight) also depends on how the SeaRey is registered and operated. An Experimental Amateur-Built SeaRey might have a maximum gross weight up to 1,500 lbs, perhaps even more--whatever the builder specified. An S-LSA SeaRey (factory-built), on the other hand, is limited to 1430 lbs. (I think. Check my memory!) And if the pilot is flying according to Sport Pilot rules (meaning a "drivers license medical") the SeaRey must be registered at no more than 1430 lbs. (The BasicMed weight limit is far beyond that for the pilot, and there's no limit at all if you have a 3rd class medical or higher; but the aircraft registration limits the maximum permissible weight for that aircraft.)
Okay, you see that the useful load varies widely. So are you too heavy for a SeaRey? Nah, of course not. But let's say it's a 1,000 lb empty-weight LSA SeaRey and you weigh 205. That's 1205 lbs to start with.
Remember that the FAA says you need at least 30 minutes of usable fuel in the tank OF ANY AIRCRAFT in order to take off in daylight and Visual Flight Rules (VFR. Not IFR.) So to take off in your SeaRey (which burns about 5 gallons an hour when cruising and probably about 7 or 8 gph during takeoff) needs to have at least 3 gallons in the tank just to take off, fly around to the other end of the runway, and land immediately.
Gasoline weighs roughly 6 lbs per gallon. So add 18 lbs of usable gas to that 1205 lbs = 1235 lbs. Great! You can fly that 1430 lb LSA SeaRey.
You can even carry a passenger who weighs 1430 - 1235 = 265 lbs. Okay, but let's get practical: Suppose your passenger weighs in at 205, just like you. How long can the two of you fly?
Whoops! 1235 (the airplane, you, and minimum fuel) + 205 (passenger) = 1440 lbs. That's 10 lbs too much. The passenger will have to go on a crash diet. But let's say the PAX is only 170 lbs. That's 1235 + 170 = 1405 lbs, 25 lbs under the 1430 limit for LSA seaplanes. So you can add another 4.16 gallons of gas and fly for almost an hour.
If you get down to 170 lbs, too, then it'll be 1,000 lbs (SeaRey) + 170 (you) + 170 (PAX) = 1340 lbs. Then you have 1430 - 1340 = 90 lbs for gas. That's about 15 gallons--less the 2.5 gallons the FAA says you must have remaining when you reach your destination, about 12.5 gallons. So you and that passenger can plan to fly for 2.5 hours. (That is, you can fly for 3 hours, counting that half hour reserve that will be in the tank after 2.5 hours.)
Want to fly longer (or farther)? Lose more weight. Lose that 170 lb passenger and you can top off the biggest SeaRey tank and still carry some baggage.
No politics here, by the way. That's implicit in the one rule, which is "Be nice." (That same rule protects you from flak about weight. Congrats on losing 45 lbs!)