You clearly have to meximize the time spent on the different islands you'll visit and not spend it on a ferry. This, though, does not apply for short distances such as, for example, Rhodes to Symi or Tilos. Flying via ATH is often the fastest way, unless you get one of the rather rare inter-island flights that Sky Express (and few with OA) offers. Greek Easter is on May 5 next year, it will be easier for you to find seats on domestic flight in April. Finally, my personal experience with Internet ferries sites is that they are often stupidly designed and, worse, the info provided is often outdated or simply wrong. Unfortunately for you, the easy solution to that for me is simply going to the next local travel agency on the island (I live on one ).