We spent the last two days of our holidays in Iraklia last year. After ten days in Naxos, we took the ferry and went to Iraklia (only 1 hour away). We hadn't booked a room but it was easy to find when we got there. Of course, it was early June which made things easier. To tell the truth, I don't know if we could book a hotel from the Internet (I don't even know if Irakliahas internet connection, even phone calls were bad!).
Not many visitors had arrived when we were there. It was obvious not only from the hotel but also from the beaches... The first day evening, we were the only people to swim in the beach! Our room was close to the port. Didn't see anything from the island than the port and the beach of Livadi, where we went on foot. We stayed only for two days and didn't have much time, prefered to relax than walk under the burning sun.
One thing impressed me on Iraklia: the hotel onwer told us to use water wisely because it could run out and then we wouldn't have water for two days!! Can you imagine that? We understood that this is the usual practice with the small Greek islands: tankers come in summer and fill in cisterns with water. Can't you find another solution for this? It seems unworking for me. Maybe, you should built underwater pipes from Athens to provide islands with water.