Fortunately, we were not in the middle of the lake when it happened. We boated to my buddies dock to pick them up, and just heard a loud grinding when we attempted to start it.

We examined the starter - it was loose, and then we found a 6 in bolt in the bilge. It was broken off right were the threads end. These bolts are vertical, and go in from underneath. The bolt closest to the block is the one that broke. I believe what happened is they came loose, and then the starter was able to torque it enough to break it. So, lesson here folks, check your starter bolts when you do maintenance.

I was able to suck down the bolt that was left just enough to start it (once), so we ran it to the boat ramp and put it on the trailer.

So, first question, these appear to be specialized bolts. There was some sort of knurling, or something, near the threads on the bolt shaft. Also, I have to assume that the threads are still left in the block. I think they'll easy-out just fine, but I'm afraid the unfortunately end of this story, is that the block has to be pulled to get that done. This really isn't what I want to be doing at the beginning of our short boating season here in Montana.

Does anyone have anyone magic up their sleeve for this fix outside of pulling the engine? If not, how much trouble am I looking at to pull this thing? I've pulled engines from cars, never a boat.... it seems like the boat should be easier in some ways. With the swim deck off, I do have decent access to both sides of the motor. I do have a large hoist big enough to do it. I'm debating on just paying a shop though.

You can buy automotive starter bolts at any auto parts store. As far as removing broken bolt try to weld a nut on the piece that is left in the block. Sometimes you can weld just on to the broken piece and build it up to be able to get a nut welded to it. It may take a few tries but the heat from welding helps loosen the broken piece,

Full size old style starter? Get the bracket that bolts to the front of the starter and bolts to the block near the oil pan rail. 1967-1969 Chevrolet Corvette Genuine GM 427 Big Block Starter Motor Bracket | eBay they used to make big block starter noses out of iron cuz the aluminum ones would break.

The starter is like this one:
https://www.amazon.com/STARTER-MERCR.../dp/B00CX5SNEA

The bolts are vertical, and install from the bottom.

starter bolts have a knurel on them to help locate the starter.i would get another starter bolt and compare the length with the not broken one you have because some are longer than others and you don,t want the bolt to bottom out.i have had issues with some starter bolts recently that were real soft and would easily bend or snap off.you may get lucky and be able to un-thread the broken one without needing to lift the engine,but if your luck is anything like mine that won,t happen

I"m going to give it a go as soon as I have time. Maybe luck will be on my side, but my luck is similar to yours. With the starter out, I'l have another 4 inches or so of space to work with, so maybe. It depends on if the threads are jacked up enough to bind.

I may end up getting a new starter with new bolts, brackets, etc... It would be nice to find some replacements bolts made of high grade material.... like grade 10 even.

ARP makes bolts for some starters. Take a look.

if that bolt is broke flush its gonna be a PITA to get out, all depends on access......the OEM Merc. bolts can be found on ebay

Ayuh,..... I've got alittle,....

Ya need a battery powered small right-angle drill,.... a little piece of 5/16" wood dowel, 'n a set of left-handed drill bits,....
Mirror, 'n flashlight are always welcome,...
Ya drill through the length of the dowel to keep the bit centered on the broken bolt, which is 3/8",...
I've got an ole set of Snap-On left-handed bits, 'n matchin' easy-outs,.... rarely to never use the easy-outs on starter bolts,...
Just the power of the drill bitin' in is usually enough to run the stub out,...

Now, bein' ya already got a little permanent magnet starter, I'll note an issue I ran across a couple years ago, when I started swappin' in the little starters,...
Ya Gotta use new bolts, For that PM starter,...
The traditional Chevy starter bolts are ever so slightly longer, they bottom out, before the starter is tight to the block,...
Which of course leads to busted starters, 'n busted starter bolts,....

That said, if the old bolts were used to mount that starter, yer probably in for a Battle, as they're bottomed out, it'll be a fight to break 'em free,...
Once free, they usually run right out on the left-handed drill bit,...

I just bought that same starter, and it came with 2 bolts and a big warning about how they must be used in place of orig bolts. It would be nice if they did up a front bracket and included that too. Maybe I will cob one up and market it...