Visit this link
https://www.catalina36.org/forum/technical-discussion/best-procedure-coo...
On this website. View the last photo that I have posted.
Basically remove the fill cap for the antifreeze fill and fill as much as you can. Then close the cap. Fill the reservoir with antifreeze.  Turn on the engine for 10 minutes. If temperature goes close to 170F turn off the engine. To burp, loosen the temp sensor on the top of the engine to release any air trapped in the engine block. When you see antifreeze leaking out, tighten the sensor.
Check the reservoir add more antifreeze if needed.
Turn on the engine. Check the temp. If over normal expected temperature,  then repeat the burping steps.

I really think you should add the antifreeze that you lost.
Make sure there is more than half of container antifreeze in the reservoir and engine is cold. Remove the temperature sensor at the top of the engine and using a new and clean srynge insert antifreeze in this opening until it starts to pour out. This will also expelled any trapped air in the engine block. Reinstall the temperature sensor and turn on the engine until the temperature comes up to normal operating temperature. Inspect if there is any loss of fluid or bubbles from the temperature sensor, tighten if needed.
After running the engine for 30 min with no overheating, you have fixed the problem.

Depending on your boat #, if yours is like mine there is a bleed valve near the thermostat that you can use, instead of removing the heat sensor.

Harbor Freight sells a $7.00 orange plastic transfer pump that I've ignored for years, assuming it is junk. Turns out it is perfect for filling the coolant system and I imagine many other jobs on the boat.
FIrst I used it pushing air to get all the water and old coolant out of the heater circuit, then with the pump input in the coolant I slowly filled it and reattached the hoses to the thermostat fitting. Then I filled the engine at the pressure cap with a regular funnel., let it sit for half and hour or so several times with the cap off, each time adding a bit more coolant with the HF pump. With the cap back on and the overflow tank still empty I ran it to 160 degrees a couple times, letting it cool and adding a bit  it more coolant until it was almost completely full.
The last step was using the pump to fill the overflow tank. After a couple hours running the engine I used it again to top off the overflow. (Once the overflow is filled don't open the pressure cap, coolant will spill out.)
No spills, the job was easier and neater than previous attempts. Next trip to Harbor Freight I'll pick up a spare pump, it is small enough to easily store and cheap enough to carry a couple.