Heater questions.....
Ok, one of the heaters we mentioned is a coleman catalytic heater "Sport Cat" that can be found at camping stores and runs on the small disposable propane bottles.
Correct, we have extended our bimini and dodger with additional canvas screens/glass/panels that snap and zipper securely. Although expensive, it has been a great investment, creating a whole additional "room" on our very compact 25.5 boat. The enclosure is not 100% sealed such that oxygen levels are a concern. There is enough "draft" between snaps to keep air properly vented for the heater.
The byproducts of properly burning propane are WATER and CO2 (that's the same stuff WE breath out). You will notice increased condensation in the boat if you are using lots of propane, but the gas is not "poisonous". The carbon monoxide will not be a concern as long as you have a proper burn and proper ventilation.