Sorry, cant see pic.
Is it a TRS or Bravo boat?
If TRS, is it a top mounted starter?

If the starter doesn't turn and the click you hear pulls the battery voltage way down, the armature or wiring to the brushes or the solenoid is dead shorted.
Pulling the manifold is the only way I know to get any working room back there. Two vertical bolts, 9/16ths head size, one large nut, could be 1/2 or 9/19 and one small nut, 5/16 or 11/32nd to remove the wiring. If the bolts wont turn, try giving the heads a pretty good RAP with a heavy hammer straight on. Often, they are seized to the aluminum housing and whackin them will break the corrosion loose.

thanks airpacker.
so next step is deciding to pull manifold, or engine.....

I suggest using a 6 point socket or wrench on those 9/16" hex heads.

You need a flex ratchet. Maybe a small length of pipe over the handle for some leverage. INSTALL NEW STARTER WITH ANTI-SIEZE PASTE ALL OVER THE ENTIRE LENGTH OF THE BOLTS. So next time you wont have a huge fiasco on your hands.(if only Mercruiser cared so much !) I would recommend a lightweight gear reduction starter for this application. BBB

My problem was that there wasn't enough clearance around the one stuck bolt to be able to slip a socket over it and it is so deep in the engine room that it's hard to get leverage. The other one came out fairly easy though.

I'd go ahead and pull the manifold. You'd probably want to do that anyway if you pull the engine to give a little more clearance, so why not pull it first and see if you can get the starter out with the engine in? The only cost of doing that is a new manifold gasket...

I have the same problem reaching the starter on my Cig. If I ever pull the engines again new bell housings with top mounted starter positions will be installed.

Good luck.

Don't forget to drop a wrench, bolt, or SOMETHING down into the bilge while your in there. Always makes the job SO MUCH more fun trying to find/reach the little stuff in back under the engines!

That way you will have the supplies handy when putting it back together.

really now, I hope it goes well. It really sucks when something as simple as changing a starter turns out to be a big ticket item i.e. pulling an engine. I like expresscat's idea.