Having engine Issues

I have an 88 wellcraft with a OMC 4.3 rebuilt engine (Stern drive). New lower shift cable, rebuilt 2 barrel carb. New points and condenser. At home the boat fires right up and work great. When I took it out 2 weeks ago on lake Tahoe NV, it was cruising about 10 mph and the engine died on me. And I couldn't get it started again. I turn the key and nothing happened. All of my gages and accessories worked fine so I knew I was getting power. The engine just wouldn't crank at all. When I got back to my house, I couldn't find anything wrong. All my wires were connected, fully charged brand new battery with new terminals and wires. The engine just wouldn't crank over. With the key in and turned to run, I finally put a screw driver directly on the hot and neg posts on my starter and it fired right up ad cranked right over, no problems. So I cleaned all the connections on the starter and used super fine sand paper to clean the posts and nuts, put on new star washers. The engine cranked right over every time I tested it throughout the week. I took it out his weekend and it was running fine until I turned it off to let my kid try the skis out. When I tried to start it, no crank. Full power to everything, but the engine wouldn't crank. I had to use a screwdriver on the starter again. Does anyone have any idea what could be wrong with it?

You of course checked the 50 amp fuse or circuit breaker back at the engine, yes?