Aside from the boat question (and David has given us a great primer on that), for the business of what to drink on land -- the accurate answer is "It all depends." Frank II says all his friends in Greece "On the mainland and in the islands" drink bottled water at home. Well... that takes in a whole lot of territory! In many places, like bigger cities, the water is technically "safe" but chlorination makes it taste less than great... and in some very dry islands, desalinization is required -- and that never results in palatable taste. In other places, as mentioned (mainly in the mountains), the water source is outstanding, and is Far better than the bottled. Some examples I think of:
• DELPHI - The pitcher of tap water I drained, at a little sidewalk taverna, came untreated from snow-melt atop the famous Mt. Parnassus I was looking up at... and was the best water I've ever tasted in my long life.
• SPILI, in central Crete - This little village, in foothills of mighty Mt Ida, has a famous Roman-built fountain with 12 lionheads spouting(1) , where tourist refill their canteens. While we were doing so, we saw local waiters filling pitchers for use at table, and local residents filling large jugs for home use. On another side of Mt Ida, the village of ZAKROS has one of Greece's biggest water-bottling plants, because the local snow-melt water supply is so delicious.
• On smaller, fertile islands with good local spring water from the heights, for many years there were a few public "taps" with spigots. Naxos has very safe municipal water fromthe tap, but taste is just meh. In the past, some free "street taps" of local spring water were used by residents, and by restaurant waiters, to fill pitchers for cooling. However Naxos' increasing tourism (a has really stressed the island's water resources (causing hardship for its farms in the hills), so I wonder whether the local faucets are still in use.