While I appreciate we're discussing hypothetical DND scenarios, I'd just like to take a moment to highlight the dangers as it's an actual risk we deal with in reality. (Cars and heating systems)

CO is a very dangerous gas for a reason. Its known as a silent killer because while there are symptoms, it's mild and easily mistaken for other medical issues. Its not uncommon to be wrongly diagnosed by medical professionals. CO works by binding to hemoglobin 200% stronger than oxygen and stays bound for hours. So unlike nitrogen hypoxia, where once you brought back to a high oxygen environment, you will start recovering, CO poisoning remains. Even a 100% Oxygen environment will not in itself save you from severe poisoning because the CO is so strongly bound to your blood that oxygen cannot displace it.

Anyways in a (near 100%) flooded environment, the players are already dead when they feel the symptoms. Once CO is bound to their blood stream the only treatment is hyperbaric oxygen therapy.

Good to know information:

0-9 ppm CO: no health risk; normal CO levels in air. 10-29 ppm CO: problems over long-term exposure; chronic problems such as headaches, nausea 30-35 ppm CO: flu-like symptoms begin to develop, especially among the young and the elderly. 36-99 ppm CO: flu-like symptoms among all; nausea, headaches, fatigue or drowsiness, vomiting.

100+ ppm CO: severe symptoms; confusion, intense headaches; ultimately brain damage, coma, and/or death, especially at levels 300-400+ ppm