Tbh, I'd rather stop her falling overboard than think "oh well at least if she falls overboard I'll notice". If I could safely stop her falling overboard, I'd pay hundreds quite happily. On top of the usual immersion risks, she has severe epilepsy and chucking her suddenly in cold water could trigger a seizure, which means I'd have limited time to MOB her and get emergency meds in.
SPR - thanks for the mention of sailability - stupidly I had no idea they did RIBs as well as sail, I'll get onto them.
Meantime, I have had another low-tech idea, which is a hybrid of the seatbelt and lash-her-to-an-adult approaches. I could fit a lapbelt which is only fixed at one end - a strip of seatbelt webbing, really. And the adult sitting next to her could sit on the loose end of it. If it all goes aubergine-shaped we're both floating free, but meantime the adult has at least one hand free to hang on themselves.