Marine starters and truck starters share the same DNA but unless you want to experience the dreaded "boom" use a marine starter -- also known as "ignition protected" as a replacement. You are also assuming "expensive" vs "free" in the diagnostic area. To spin the engine a starter must be in good shape, the solenoid must transfer power to the starter, the battery cables must carry that high current and the battery must be able to supply it. Finally, the ignition switch must be able to energize the solenoid to make all of this happen. So why not start at the source rather than the end of the chain where all the expensive stuff is. 1) Is the battery fully charged and in good condition (you know that for certain???). 2) Have you disconnected the battery cables and cleaned the terminals (make them shiny clean and bright?) Remember the cables have two ends -- one at the battery and another at the engine. Doesn't do much good to clean one end if the other is the problem. If the problem is not fixed at this point use a short piece of wire and jumper the small terminal on the solenoid to the large post on which the large battery cable FROM THE BATTERY is connected. If the engine now spins reliably both the starter and the solenoid are good and the ignition switch circuit is bad. If the symptoms remain the same THEN you can suspect the solenoid or starter. It is now time to break out your voltmeter. If you don't have one, any electrical dept at a Home Store will have one for less than $15 and if you own a boat and intend to do any repairs, you need one. Set the meter to read 12 volts (probably set the know to 20 volt scale if this is not an auto-ranging meter). Place the POS probe on the small terminal on the solenoid. Place the negative probe on the engine block (unpainted area). Have someone turn the key to START. If the meter flickers rather than showing a solid reading you have an ignition switch/circuit issue. If 12 volts appears that circuit is ok (as you described, the solenoid should at least click during this test). The last test is to run a jumper cable from the positive terminal of the battery directly to the large post on the starter (not the starter solenoid). Be ready for a spark and don't be bashful making this momentary connection. If the starter spins, it is good and you need a solenoid. If not, you need a starter. And replace the solenoid at the same time.
