I have a pontoon with a 75hp Mariner. When I turn the key the solenoid always clicks but the starter doesn't always turn. It can take 4-5 times of turning the key back and forth before it engages. My first thought was bad starter but when I run power straight to the starter it cranks fine every time. I swapped the solenoid but the old one was fine. The bendix cycles up and down free and all of the connections are good. I check the switch and it is good too. Anybody have an idea as to what else it could be?

If it works every time you bypass the solenoid with a jumper, the problem is somewhere in the solenoid, or the cable which feeds power to the solenoid.  A solenoid is nothing more than a magnet created by the electric current.  It pulls down on a jumper bar with contacts on each end, activating the starter.

 

After much use, the contacts get coated with carbon from arcing, which acts like an insulation.  It gradually builds up as the contacts arc, until it does not always make a good connection.  If after swapping, you get the same intermittent problem, follow the line from the solenoid to the battery.  It's possible you have a loose or dirty connection at that point.  Take it apart, clean it up, and reconnect.  See if that solves the problem. 

 

Did you jump from the battery directly to the starter.  If you did, then try jumping directly across the two connections (battery input, and output to the starter).  I'm guessing that you'll have the same intermittent problem.  If you do, then the problem is with the cable from the battery to the solenoid.