One click and lose power

[CMStamp]

Hopefully someone can help. Haven’t ran the boat in about a month. Yesterday I went to start it and get it running and all I heard was a click then lost power to the dash. Turned the key off, turned it to start (had power to dash), then when I went to turn it over I heard a single click then lost power again. I checked the battery, any lose wires, all fuses, put in a brand new starter and spark plugs two days before and cannot figure it out.
Could it be the ground wire? Using the multimeter I put the neg end on the engine ground location and pos end on the battery terminal and it read 12.7.

 

[Rick Stephens]

Sounds like a dirty connection to me. If in doubt, pull them apart and wire brush until shiny.

 

[Bt Doctur]

Remember there are 2 ends on each cable!

 

[Bondo]

Ayuh,..... Clean to shiny metal Clean, both ends of both battery cables,....

 

[zellerj]

Voltage doesn't mean much in a start circuit, because a high resistance connection will also show a trickle of voltage to a multimeter but will not allow enough juice to turn the starter. Will it start if you jump the solenoid?
Did you lean on the 10 pin connector while r/r the starter and break a connection?