This was really helpful, thanks all.
I did talk to Le Boat today and as expected the number of people they allow is the number of beds. It's not that I'm looking to take unnecessary risks or break the law or Le Boat policy. But I feel better informed having got the common sense check from experienced boaters here.
Interestingly, they tell me that there's no difference between a toddler or a heavyweight when it comes to counting people, and that I can take dogs of any size, and they don't count as people either. So there's little logic or actual compliance to float testing happening here, just policy.