Here's a response I posted on Sail Net last year....
Just because the engine is "up to temp" it in no way means the t-stat was fully open especially if at a dock idling with no load on it. T-stats open and close quite a bit and those temp springs are nearly always in movement unless at or near full load and pushing up against max design temp/flow etc. which may or may not happen at idle with no load. Unless you absolutely know the t-stat was open I would not risk it. Pulling the t-stat is good practice.
If the t-stat not fully open then the antifreeze can just by-passed the block and still come out the exhaust. I have seen many a RWC engine that froze due to improper winterizing. The only sure fire method to know you got full circulation of the antifreeze is to remove the t-stat, reinstall the housing without it and run the engine. This is fairly standard practice when winterizing a RWC engine.
On the 2 GM10 the coolant path looks like this
Path 1: Seacock, raw water pump, bypass junction, injection elbow of exhaust
Note: There is always water flowing out the exhaust on this engine despite the orientation open/closed of the t-stat. So just because you see coolant out the exhaust does not mean it went through the block.
Path 2: Seacock, raw water pump, bypass junction, engine block, cylinder head, t-stat, exhaust injection elbow.
If the t-stat is not open the cooling water completely bypasses the block and flows straight to the exhaust. The water delivered from the RW pump is basically branched into two directions at the cylinder intake coupling which is nothing more than a tee fitting. One side goes direct to the exhaust, so you don't melt the hose waiting for the t-stat to open, and the other goes into the engine & when the t-stat opens and allows flow to cool it. T-stat not open or only partially open means no or only minimal flow through the block.
What type of antifreeze will you use? Propylene glycol / (PG) should not be diluted so be sure to suck enough through. You may also want to use the -60 or -100 PG stuff that is intended for engines which has a better anti-corrosive package in it. It is usually blueish or purple.
While you may sufficiently protect the engine without pulling the stat you also may not.
One trick I use on the RWC 1 & 2 GM's is to pull the stat then pinch the hose between the block inlet Tee and the t-stat housing using a pair of needle nose vise grips. This ensures that the the antifreeze I am pumping will NOT take the path of least resistance and flow direct to the exhaust. The pinching of this hose FORCES the antifreeze to flow through the cylinder block and head BUT only with the t-stat removed. DO NOT pinch this hose with the t-stat in place or you could melt your exhaust system.
In theory draining an engine SHOULD work but it still has a decent failure rate and a high potential for internal rust, especially on a RWC engine. These engines do not always sit level in boats, as the guy in Japan designed the drain around. Water can get trapped and may not all drain.
After many different NE boat yards I have yet to come across one who is willing to drain engines as standard practice for winterization, unless it is an outboard. Even in fresh water systems my yard makes you sign a waiver if you want your system "blown out" because it has proven to be unreliable on many boats, especially ones with large domestic plumbing systems.
Marine engines often sit at odd angles, have drains at that are not at the bottom of the coolant passages, do not capture all the water in the block nor drain it and thus some can still freeze up when drained. While some engines do drain well but many don't.
Take the M-25 / Kubota for example this popular engine has multiple locations from which one could drain off the block yet you still don't get it all. I have flushed many of these engines and even after a thorough draining there is still a lot of dirty antifreeze remaining which on a RWC version would be fresh water that could freeze.
Pulling freeze plugs can often get more but this is the last thing you want to do on a rusty old marine engine as they will rarely if ever re-seal even with new freeze plugs. They can also be next to impossible to remove/re-install with the tight conditions on a boat.
Pulling a t-stat on a RWC engine takes about four minutes and on a RWC engine it is a good idea to replace it yearly anyway if it is not in good clean shape. I pull mine every other winter because I replace the antifreeze bi-annually. I prefer to flush my engine without the t-stat in place as it makes it far easier.
Over many years I have seen some serious crud buggering up RW t-stats as raw, dirty ocean or lake water has to pass through the t-stat, & block, to make it out of the engine. Normally I replace them. Engines are very expensive so it pays to winterize them well. Properly filled with the right mix of antifreeze I have never seen one freeze. I wish I could say the same for "drained" engines or winterized RWC engines with the t-stat left in place. The 1GM is an engine that actually drains somewhat well, better than many, but I have seen some installations that I would still feel quite uncomfortable with even if you ignore the high potentials for rust internally.
Even if you own a FWC engine that has a HX you should test the antifreeze in the block to make sure it will not freeze. I have also seen numerous examples of boaters who did not know they had a leaking HX, one just last weekend. The leaking HX diluted the engine antifreeze and the engine froze in the winter. A one minute check with an antifreeze tester, before putting her to sleep, will tell you if you are safe for winter.
Hand types faster than the brain...