2006 90 Mercury boat. Will crank but won t start?

Me and my father in-law went out fishing in his boat that has a rebuilt engine. Ran beautifully and were out on the water for 8 hours. He had taken his son out days prior and had no issues. We had just replaced the 3 prop to a 4 prop (measurements were exact). We were coming back to recover the boat and we had to... show more Me and my father in-law went out fishing in his boat that has a rebuilt engine. Ran beautifully and were out on the water for 8 hours. He had taken his son out days prior and had no issues. We had just replaced the 3 prop to a 4 prop (measurements were exact). We were coming back to recover the boat and we had to wait for other people to launch before our turn. Only took around 10-12 minutes while we sat there with the boat idling. We brought the boat close to shore and I held it there with the engine turned off while he got the truck. He got back on the boat and tried to start it and we got nothing. It would crank but would not fire up. Manually recovered the boat and drove home. He suspected the engine to be flooded. Waited the next day and it still wont start. He checked the spark plugs and they were clean. Fully charged the battery and has a full tank of fresh gas with the required mixture of marine oil. Any help or suggestions?
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