are you getting fuel down the throat of the carb?... if this isnt happening, it wont run.. you can pour a couple tablespoons of fresh fuel down the carb, which if its getting good blue spark, it should fire and attempt to run until it burns off the gas that was poured in to it..... be careful with this procedure, as sometimes you can get a back fire which can catch the gas container on fire, and then the boat.....
I use a squirt bottle for this, NOT a spray bottle, but a bottle with a small hole in the lid that can be squeezed to dispense the liquid.... and KEEP IT AWAY from the engine while cranking....
if anyone has worked on the boat without it ever running again, you need to check and see that the plug wires are in the correct order from the cap positions to the correct cylinders.. and you should see blue spark at the spark plug when checked with the plug out of the engine, inserted into the wire, with the metal spark plug body held firmly against the engine block plug...... while cranking. if its yellow/orange spark, then something isnt adjusted right and it may not be firing under pressure in the cylinder...
its possible that if someone worked on the boat in an attempt to make it run (without knowing what they were doing) the timing could be off also....