The advertised gph specs on pumps are with no hoses attached, and zero lift. The performance drops as the lift height and hose length increases. You're going to have 2-3 ft of lift minimum to get up and over the gunwales. I remember it taking about 10 min for a 750 to fill a 600 lb sack.
It takes my Tsunami 1200's about 5 min to fill a 750 lb, and that's with less lift because they're in a built-in system (but a little more hose length). You can get them from basspro.com for $37 brand new (with the elbow fitting and uncut wires / base fitting).