So far, I haven't heard solid confirmation that it's a spark issue.
Did you CHECK FOR SPARK when it won't start, or are you just GUESSING that there's no spark?

Please check for spark before you end up replacing half of your boat guessing.
Pull the coil wire off at the distributor cap and hold it close to an engine ground and crank her over.
Either you get a fat spark from the wire to ground, or you don't. Spark should easily jump 1/2" gap to an unpainted engine ground.
If not, then you have some ignition troubleshooting to do (power to the coil? trigger good? advance module ok?, etc.). If you have a fat spark, then check at the plug wire (pull one off a plug, stick a plug into the boot, hold it against an engine ground and crank it). If you still have spark at the plug, then you need to start looking at fuel related issues (vapor lock with crappy ethanol "enriched" fuel is a horrible problem these days).

But CHECK FOR SPARK before arbitrarily deciding out of thin air that you have an ignition problem.

MC

Thanks. Will definitely do that!!

No don't have a VST tank.....

Wow... Start a post and the keyboard tough guys come out...... Funny... Lets start here. Checked all the distributer components and everything checks out. Like I said before the boat runs perfect the first time you start it up. It will run for 10hrs straight in smooth or choppy water without any indication of electrical or mechanical problems.. ie grounds. Although once someone turns off the motor after reaching destonation it will not fire back up for many hours later. The fuel pump will prime and motor spinns over but no spark. (Yes I Checked)... I have looped checked, ohm, checked continuity, traced, tightened and verified every wire and terminal on the harness with Mercruiser book. I think the ECM, which is located on the exhaust riser, is getting warm while the boat is running and loses some signal once power is turned off. I am not getting any potential from the ECM to the distributer with the key on or while motor is spinning on starter. I wanted to know if anyone has heard of this before? I dont want to change the ECM just because... Very expensive. Have plans to barrow another ECM for testing.

I have an update to this problem... It has been fixed. The intake valves in the heads were not closing fully in the seat which was allowing the fuel to puddle on the piston. Seems that after decking the head and working the valves it causes the intake valve to sit a little taller on the spring. The machine shop dusted the valve to correct the length but not enough. Since the rocker stud is torqued instead of having adjustable locks there is no way to adjust for preload on the lifters other than shorter pushrods, taking more off the end of the valve (pulling the head), replacing the roller rockers or shim under the rocker stud to meet the longer valve. We shimmed the stud for a temporary fix till we change the heads this winter. Thanks for the help and hope this will help someone later......

Why not just install some rocker arm studs to make them fully adjustable???? http://www.summitracing.com/parts/arp-135-7102

so valves being to tight stopped it from firing ???...good luck with that !

I have to agree. I wouldn't take it apart for that reason unless it needed to be. Although I would think it needs a quick vavle job to clean up the seats.
If you are running stock valve covers you will need to use shorter lock nuts like http://www.summitracing.com/parts/crn-99792-16/overview

What I gathered from how he put it was decking the head made the valves sit up higher and then they were loose in the springs so they didn't seal off.
Not that decking a head has anything to do with shimming valve springs properly, but any spring pressure should have sealed the valve onto the seat.
Now if the valve was bent or galled in the guides that is a different story.

the op stated that when the engine would not restart he checked for spark and had no spark,thats a far cry from tight valves if you ask me.it looks like more than one issue with this one.