what is the source of that information, for if it is true it means than more
than 3 times as many people die of CO poisoning each year as die of ALL
recreational boat accidents causes combined?

You can verify the statistics independently
through a search engine. There might be 100 sites with the same statistics.

The figure of about 2500 remains fairly constant for North American (US and
Canadian) deaths each year for CO poisoning from all sources, (not just boats).
There are more CO deaths from
motor vehicles, space heaters, etc than from boats, but the boat number is
disproportionate when one considers the
hundreds of millions of people using automobiles, cooking appliances, and space
heaters on a given day vs. the tens of thousands actually underway in a
motorized pleasure craft at any one time.

CO poisoning is the leading cause of
accidental death by poisoning in the United States. Not all exposures are
fatal, but the severity of the problem is also illustrated by the fact that
10,000 to 15,000 people will require medical treatment or hospitalization for
CO exposure in a typical year. (From all sources, not just boats).