The basics. Been the same essentially since the days of the Model T
compression, fuel and spark
1....Compression. Engine must have adequate compression, AKA not worn out, timing (cam drive) not "slipped" nor cylinders washed down with fuel or dry from extended storage. Run a compression / leak down check
2....Fuel. Enough, but not too much, and FRESH. NEVER assume that "what is in the tank" is going to run. Check plugs are they dry, wet?
3...Spark. Good hot spark and in proper time. If possible, check with a WIRE solid core coil wire and a test gapl Cranking with a good battery, should result in a nice hot blue spark "in rhythum" at least 3/8" and typically more like 1/2"
Modern fuel sucks and can cause vapor lock/ fuel boiling problems in carbureted cars, likely "not so much" boats as the engine bay is many times much cooler.