86 Supra 454 won't start

We have used the boat this season, it wasn't starting as quickly or consistently as last season but it was running. Had a failure to start and changed out the key switch (ignition) and it ran again. last time out, nothing. I tinkered around looking for loose wires or connections would try turning it over and would get the clicking noise that I believed to be a bad starter. I pulled the starter and brought it to 4 repair shops and 3 auto parts stores, all said except for one " what is that thing, and what was it in, thats huge". Last NAPA guy knew as soon as I put it on the counter (thank god for rednecks). He put power to the starter and got it to spin, tested the solenoid and said it seemed good. I bought and replaced the cranking battery and solenoid just to be safe (I am an hour away from that NAPA). Now It sounds like the starter is trying to spin but the engine isn't firing. I pulled and plug and held it to the manifold and no spark. The spark plugs are in bad shape and I am going to replace them when I go back. Now I am looking for some troubleshooting help/advice/prayers. Thank you in advance for any help!