Having done the run to Kangaroo Island many times myself in the same aircraft... its nothing out of the ordinary. I have also taken Socata Tobagos across the Spencer and St Vincent gulfs at night, with life jackets on. Almost all of the KI runs from Parafield done by FTA are IFR, with life jackets worn (Company requirement due to the approaches at Adelaide that normally start/end a flight) under constant surveillance by ATC.
Not once have I been concerned about taking a single engine aircraft of that type over water, they are maintained to a very high standard - and I am only aware of one aircraft with the same engine type having engine issues within that company in the last 5 years (Rough running). In the words of my old instructor: "The aircraft doesn't know it's over water... It's no more likely to fail over water than over land"