Dangerous for two reasons:
The CO2 buildup is something you can work around, if you INhale through the snorkel and EXhale out your mouth or nose--so you don't exhale used air back into the snorkel.
But, lungs are also more fragile than you think. There is some slight risk that if you have a thin spot in your lungs (some people do, there's no way you'd know) the extra effort of sucking the air in could blow it out. Note that I say "slight" risk, you're not pushing it much.
When I was a kid they used to sell 3' long snorkels for kids...that's like trying to breath through a straw, because of the extra water pressure on your lungs. Every foot makes a difference.
CO2 problems have been getting a lot of press this year as kids are dying from "shallow water blackouts", i.e. they are flushing their lungs and then doing breath-holding contests under water. If you flush the CO2 out that way, your body can also fail to sense the normal CO2 buildup and you wind up passing out for no obvious reason--and drowning. Not your problem, but one of the complications when land mammals try to be marine mammals.
BTW, a plain wool watch cap makes good protection for your head. No added buoyancy, some added insulation, some added "banged my noggin on the hull!" protection too. It isn't armor by any means--but it HELPS.