mercruiser 350 inboard starting issues

[chance1234]

Good afternoon gents, This is my first post so i should introduce myself. I am in the coast guard in boston and last winter i picked up a 23 seacraft (78) with a 1995 or 96 (?) mercruiser 350, 260hp straight inboard. I have numerous problems with her so far but have fixed all but this one. I thought i had all the problems solved and went to put her in for the year. She would not turn over, it sounded like when a bettery is dying (turning over real slow) so i pulled her out and tryed on dry land. Now all she will do is click when i turn the key. I put a voltmeter to the batteris and they both read 12V. I then replaced the starter, starter selonoid and slave selonoid and still the problem persits. It is the slave selonoid that is clicking and nothing else is happening. i have checked all the grounds that i can find and everything seems good. Isin't there supposed to be 1 main ground from the block to a thru hull? If so i see the thru hull but there is nothing going to it. This really frustrates me as i paid a "professional" to re-wire the entire boat this off season. I was told this is the place to go for stuff like this. Sorry this is so long and i will keep them shorter in the future. Thank you for any help.

Elijah