The sea varies by day so it's a difficult prediction job. Most days it's not rough in June, but it is easy to hit a period of rough sea for several days.
To avoid the direct route small enclosed catamarans you can go walkabout and take larger ferries via other islands, actually advantageous as you see other less tourist saturated islands in the process.
Another way is to minimise the use of the small catamarans and take it for 45 mins to Paros and again get the Bluestar ferry to Santorini.
There is a vast array of differing ferries/seating too complex to detail. Just buy the cheapest ferry ticket and 'manouvre' once aboard.
OK I misread the direction, but the principle is exactly the same in reverse. There is no mention of same day connection.
There are a number of early morning ferries from Santorini some days of the week NEL at about 09:00.
Some people do really feel very bad on these small catamarans, and they will do anything to avoid them.Edited: 26 April 2013, 16:49
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