boat was getting on plane(1993 merc 502)and engine made a small backfire/cough,then a minute later the tach went bad(bezel glass filled up with smoke)went back to dock to check things out,opened up dash panel and did find a wire came off one of the other guages.went to start back up and one off the batteries was 100% dead(was the weaker of the 2 batteries).weird that all went bad at same time?

No coincidence. Sounds like either a hot lead or a ground came loose and shorted out. That would cause the ignition to go crazy and make the engine sputter. That would also cause the battery to go dead.
Eddie

Did you know that using a weak battery will distroy the alternator?? get 2 new ones and fix the wire..

battery could have shorted out internally ??

thank you for the replys

If you shore something bad enough to create smoke and kill the battery, consider yourself lucky you didnt burn it to the waterline.......