My Boat engine will crank over but not start?

When I try to start my boat engine it will keep cranking until the batteries go dead. It seems as if I'm not getting any spark and yet when I pulled a plug it shows spark. I have replaced the points, plugs, coil, distributor cap, coil wire. The carb is getting plenty of gas. Any ideas? it is a 1987 Seaswirl, 4 cyl.