I doubt very much that it is a positive ground system unless it is a very old English
boat. Try finding a wiring
diagram on line and trace things out. As far as testing the starter with it out of the block, just leaning it against the block wont give it a good enough connection to show if it is working. A diesel
starter has so much torque that if you test with jumper cables
( a better way) you will have to put your foot on the starter to keep it from jumping half way across the room. If you do it with jumper cables
, put the negative to the starter case, and touch the positive to the insulated stud that goes into the starter. If it spins up slowly , or not at all, then you have found the problem. If it jumps like hell, the starter is probably fine, and check the wiring
again.____Good Luck with it._____Grant.