Sirkyboy
I don't have a difinitive answer to your question about the life jacket,but in Turkey they cater for everyone and i'm sure that they will sort something for your daughter ....especially with notice.
The boats have a 3 ft high side to them on the lower decks, so your daughter will not be able to "fall in". The boat that
ICR use "Queen" i believe, has excellent access to the water as the the slope of the steps is not too great. (If you choose not to use the ICR trip and go to another boat, check the access to the water as i have seen grown blokes struggling to get back on some of the boats)
The only one to choose really is the ICR one. The staff are brilliant. last year i was on a boat and there was little downs syndrome girl who was frightened at first, but within half an hour or so Donna had put her at ease and the little girl was even swimming it was great to see. It was Donna's first time swimming as well (remember Donna??)
Rest assured your daughter will be fine. Give it a go and enjoy
John