Not sure where you're getting your specs, but a Hobie 17 Sport is more like 340 LBS. If your boat weighs 800 LBS, then it's about time to open the drain plugs because the only way it would weigh that much would be if it's 1/2 full of water...
As for the trailer, it depends on whether it's an aluminum or steel trailer and how heavy/thick the frame is. An aluminum trailer would likely be around 200 LBS. A steel trailer would be more in the 350 to 500 LBS range. If you have a cat box, that would add about 30 to 60 lbs.
Are you trying to figure out whether or not your car has enough power to pull the boat?