No spark

test voltage at the coil?

So, spark starts at the battery, as you noted, you need 12 volts at the coil when the key is on. the cranking voltage drop is common, and some people have run an additional 12 v from the start position of the key and bypassed the ballast resistor to increase the voltage during this starting procedure...   might be worth looking at. since you already have voltage though, you can go to the next step:

Test the coil.    there are a couple of ways to test a coil, you can put a spark plug wire in the center post of the coil , connect to a spark plug and ground the end of the spark plug.   Watch to see if there is a spark in the plug gap.    You can also measure the resistance of the coil primary to ground terminal... This will be different for each coil brand so you need to find out what the intended resistance should be for your coil.   

If your coil checks out, the next spot that the spark goes is into the points (if you have points).   poorly adjusted points or corroded point contacts can stop the spark from the coil travelling to the distributor cap. file/adjust/replace the points and set the dwell.      Setting points includes having a dwell meter, if you don't have one, you can buy one on a timing light, or an engine analyzer type meter.    If you have an electronic ignition module, this could happen too, i am not sure of the process to test an electronic ignition module though.

Try these steps and report back...   there are guys on this forum that are much better at this stuff than I am... but I think this will get you pointed in the right direction

I started changing parts due to not having any spark

It's a petronix. And no gap adjustment. The other day I notices that the tach jumps around a lot while cranking. Is the whole boat grounded out somehow?

disconnect the Tach and see if your problem goes away. A bad tach can ground out your coil.
Disconnect at the coil, could also be grounded wire.