Looking at a boat. Spoke with marine mechanic who services boat. He said last Aug 1 plug on each engine (same location) had popped loose and owner noticed water inside engine compartment. He some no signs of freezing, water in oil, etc.
Said very strange, but confident in motors 500efi, low hrs.
Boat's in TX.

Does this sound possible, or should I assume the worst & steer clear?

Not a good sign. Could have drained only one side of the block.
You can easily pressure test it and check for cracks.

Would've filled his bilge in 5 minutes. You wouldn't believe how much water a raw water pump pushes.

I'd have the engine looked over real good.
And no, they generally don't just pop out.

BTW, the term "freeze plug" is wrong. They aren't there to keep the block from cracking if frozen. It just happens that they sometimes pop out when the coolant freezes.
Proper term is 'Core Plug". That's how the foundry gets the sand core out after casting the block.

all above correct in my opinion. core plugs are slightly tapered and have a sharp edge. installed correctly they are NOT flush but actually about .060 to .125 deep into the hole. this allows the edge to " bite " and prevent movement . if these are factory, they didn't " just fall out" if they have been changed at some point and put in flush then , yes... i have seen those just fall out" look at the others... that will give you an idea of whats going on.

damn it, I've looked at many boats I've turned up my nose at. Then the one I like has this issue. I can just imagine kicking myself in the butt later if this became a problem.

The mechanic said this both engines, same plug location, so he thought it may have been a factory issue. Boats been on a lake in Dallas, don't believe they winterize.

How does a pressure test work?

Pressure test on cars is simple. There's a radiator cap adapter with a pump and pressure guage. Screw it on, pump up 15psi, see if it leaks out. Same can be used on closed cooling.
You'll have to rig up something if it's raw water cooling. You can use hoses to plug the water outlets into each other, capping the outlet side. Then rig a plug with pressure gauge, air fitting and shutoff valve on one of the inlet hoses.
I've done this a couple times. There's probably a tool for it, but I had enough pipe fittings and parts to make my own for free.
Pressurize, shut off the inlet air valve, then see if the pressure gauge starts dropping.
Gary

Put in a rubber plugs and run the engines, if there's a crack you'll see the leak!

A frieed of mine had this same issue on a boat he bought, the guy thought the engines were cracked and done. He ran the thing all summer no problems.

Wish you luck,

Kirk

Kirk, I'm kind of slow.....could you please explain "put in rubber plugs". Do you mean plug the raw water exits to exhaust manifolds? And as I start engine, water will want to exit somewhere, via crack if there is one. I assume I'd only run for a very short time like this (a minute). Or am I way off?

I bought a boat a few years ago that did freeze and a plug poped out.Before I removed the motor I went to napa and bought a rubber plug,just tell them its a big block chevy and they will give you the proper size.It has a bolt and nut throught the center,you just put it in the hole and tighten the nut till it seals.Its not a permenant fix but it was only three dollars and fifteen minute of my time so I could run the motor and make sure there were no cracks or other problems with the boat.I ended up running the boat all summer with the temp plug in.

Nordic95