I have the same boat, same age, same engine. At first, my coolant level was dropping after a long run and I found that it was leaking back where the coolant enters the hot water tank. Most of the heater hose had been replaced over the years, but not a couple feet back there. The hose "sweat" when hot and must have mostly evaporated. New hose fixed that issue.
From the picture in your other post, it looks like your coolant overflow bottle is not the aftermarket reclamation type. Those are a sealed system. Your bottle seems to be only an overflow catchment right underneath the original metal overflow tube. I still have the original system and after fixing the HW leak am having no issues once I found the proper coolant level.
If you fill the coolant reservoir on a FL up to the top, it will poop out the excess. The "excess" is about 1/2 of the level in the tank. It seems that more coolant would be better, but these engines doesn't think so. I made a dip stick out of a Pop Cycle stick. Although the tank will hold 2 inches, approximately 1 inch is full and 1/2" is empty. You basically need enough in the tank to "burp" the petcock on the manifold (if you've seen how to do that on TF).