About the round-the-island tours ... I was there May 2011, so prices doubtless have gone up somewhat, but at the time most were €42-45. This includes morning coffee (most tours take off around 10 am), lunch aboard OR, weather permitting, onshore at a cove, including glass of wine, and afternoon snackies as you head back. Since you have a number of days in the island, it's prudent to check in the afternoon what the weather is likely to be the next day, and THEN book a tour. The boats are all lined up along the Adamas harborside, and people walk along before dinner, go aboard & talk to Captains & make their choice.
Here's a great map of the island, to check out http://www.seakayakgreece.com/virtualtour.htm Tours are counterclockwise and are either completely AROUND the island, or halfway then turning back. The downside of All-around is that there is so much ground to cover, there isn't as much time to stop to explore, swim/snorkel or picnic onshore. The halfway one goes as far as Kleftiko, which is the last really scenic volcanic cave/arch area ... by not doing the whole circuit you do not miss much unique water-level scenery.
The next choice is vessel size. In High Season there are larger boats, holding 30-50 people, and it's quite a party scene, but that won't be on offer in May. The 2 May choices are (1) small 1-masted (sloop?) capacity about 6 + skipper, with a mainsail & foresail or (2) a large catamaran, which holds about 14 + 2 crew. 2 of our party went with a small boat/all-round island deal, because they are sailors and looked forward to crewing. 3 of us chose the Catamaran -- name of it is a mouthful -- Chrysovalandou -- and we were the smarter/luckier ones. First, there were only 8 sign-ups so there was a ton of room to loll about on deck sunbathing. Second, it was quite windy, but because the Cat was big, and very stable, we were able to be under sail most of the time. However, the small boat that our 2 friends were on dropped its sails and they were motor-driven 90% of the way. The Milos volcanic formations at the waters edge are amazing... and cannot be seen from land. We stopped twice for swims, the 2nd time we had spaghetti and seafood sauce, yum!! and went on rubber rafts to negotiate the sea arches at Kleftiko. The water was BRISK! but I loved it. On the return lap, sun shone brightly and sunbathing (to greek music) was Brill! Stepped off the boat @ 6:30 windblown and ready for a huge dinner. Totally worth it!