Having done this trip twice & a few rivers in Scotland I very much agree with what Andy (Tinkerman) has said.
You should be reasonably fit & a confident swimmer if you plan to go rafting.
Unfortunately, accidents happen, you could be killed crossing the road!
If capsising in the raft worries you, don't go rafting!, you WILL capsise, it is part of the experience. In my opinion, if the raft guides instructions are followed then the risk is minimal. One of the most important safety issues is that if you end up in the river, (& you will) do not try to swim but instead, lie on your back with your feet pointing downstream. It is far better to bounce off rocks with your feet than with your head!
My condolences goto the deceased person's friends & family.
I look forward to rafting Dalaman river again next year.