we used the force 10 heater the first year we owned our boat...., and slept with it going, as its a vented heater. (but we did install a CO monitor/alarm) and it never killed us or even made us sick. the following winter I learned something I did not know about it.... when its really cold and you have the boat closed up, and need the most heat from it, it will shut itself off because it senses a slight drop in the oxygen in the cabin space (it shuts down when it senses a 2% drop in O2).... so you are without heat.... so even though it never harmed us, it has angered me a few times due to the low oxygen shut down device that is built into it....
if you leave the hatches open to keep the O2 levels high, the heat will rise out and you are left cold.... on a 34ft boat, with it closed up, the force 10 will run about 4hrs on high before shutting down..... (it will run longer on low)
the CO was never a problem, as I monitored the count and it never came close to the alarm level.
since then I have installed a Wallas forced air diesel heater.. and it heats the cabin nicely, with much more comfort due to the constant exchange of air... its a much drier heat. and the ironic thing is, now, due to the positive ventilation given by the wallas, the force 10 will burn on high and stay lit indefinitely... which I have never had to do, but when the wind is howling and the temps are well below freezing, its a nice booster heater..... and in my opinion, thats all its really good for.
as for the buddy heaters, we have one in our macgregor, and they are built for indoor use as long as there is ventilation because it will still deplete the oxygen.
the down side is they still create a lot of moisture that is given off by them, so unless you want damp bedcovers, you have to have the hatches open when its on.... we only use ours in the mornings while the coffee is brewing and the hatch is open anyway... when used in that manner, it may work for you, but dont expect it to be the dry comfortable heat like you get at home....