Also use nuts and washers on your cables...not the little hand tight wing nuts. I know that doesn't sound like it would make a difference but it will. Think if you just "hand tighten" your lugs on your vehicle....lawn mower....or 4 wheeler...same goes for the boat. They need to be tight.
Try pouring a little coke on the posts...this will eat away any corrosion and will give you a better connection for a bit while wet. Pour then try to start and see if that changes anything...if so...check your connections.
Just like IBFSHN said...check those wires. Saw this on a kenworth a couple weeks ago. Changed everything out...still problems. Cut up into the cable about 6"...corroded the whole way. Redid that piece...now we are all good.
Doc Holiday; "I'm your Huckleberry" "You ain't no Daisy"