The CO2 buildup would kill them long before they reach the 17.5% O2 mark. And considering how hard it is to actually walk in water, i'd say it would almost be e

The CO2 buildup would kill them long before they reach the 17.5% O2 mark. And considering how hard it is to actually walk in water, i'd say it would almost be equivalent to working out. According to the article, a person that is heavily working out would have 10 minutes to reach the 17.5% mark in a 1m^3 area. So maybe 20 minutes for the boat people? And that is neglecting the CO2.