Quoted from lester McCarthy
"Report by Dr. C.J. Brooks - Sea Survival Systems
This is worth a read reference capsizing and self-righting gear fitted to Ribs."
Worth reading if you've not done so already, they quote that there is very little air gap left in the upturned hull as Richard B pointed out !! they also say that the survival suits hinder escape from the upturned hull as the trapped air fills the legs and holds the lower torso and legs inside the upturned hull pinning the body to the sponson !!
But in conclusion they also state that there is still very little recorded information about inverting a FRB to be able to suggest at this time accurate methods for escape.
Which means in plain English if you turn it over your pretty f**ked !!