of course Calli, of course- we all have. But I do think most of us would not have allowed a young kid on an inflatable on the sea. Would you? For exactly the reason you stated.
Someone mentionned inflatables on pools - and as a trained pool safety officer, they are so dangerous on pools too. We often had conversations with pool attendants in holiday resorts who were tearing their hair out with parents who allowed their kids to disobey, or disobeyed themselves, the clear instructions that inflatables are NOT allowed. Attendants have to see the bottom of the pool and inflatables obscure this- and would hide from view any child or adult who had floated below the surface and were drowning, or indeed drowned, because of this. When they reminded people of this, they were abused and told to f* off.