The local shops in Molyvos stock swimming shoes and if you venture into the see there you will need them. On my husbands first swim his foot brushed against the top of an urchin - it's very painful and it's difficult to remove the spines. The shoes are well worth it.
We visited Molyvos in June and the weather for the majority of the two weeks was gorgeous. There was one day when a storm came over the island and there was a downpour. As Molyvos is a built upon hill the rain water drains down and it created quite a little river outside our hotel!
Especially when the August wind blows, a cold drink can easily help you to catch a cold. So always bring a sweater or coat. It's easy to catch it, because when one person has a cold, the whole town will have one.
Also, the ice cubes in the drinks can be made out of tap water. It's not that bad if you accidentally drink a bit. But it is just a warning if you wish to avoid tap water as much as possible (which seems unlikely because you have to shower right?).
You can find a 24 hour doctor in Petra. He's somewhere between machine bar and Tree House I believe. Anyway, you will recognize the green cross. He will charge you 50 euros.
There's a pharmacy in Molyvos, also in Petra. Just ask around. It's up the hill somewhere.
The water from the tap contains a lot of calcium. It's best to buy a few bottles of water and use that to drink. Brushing your teeth with it and rinsing your mouth won't do any harm. Just don't swallow.
Lesvos doesn't really have a very good sewerage. You will probably hear this on your way over, but let me be the first to tell you. You can't flush your toilet paper. You will have to throw your paper in the bin next to the toilet. You can imagine what will happen if you don't stick to the rules.
(A few times doesn't do any harm. Don't make that EVERY time.)
As you can see on the picture, the little alleys of the city Molyvos do not come with a lovely cemented road. It's a monument and they are not allowed to touch them fine rocks in the road. So if you decide to go up to see the castle on the top, even if you go out for a drink in the harbour, make sure you wear comfortable shoes. High heels will definately kill you. The hills are steep! I experienced that myself ladies.
Renting a car and driving around the island is an absolute must. Even though it is the third largest island in the Greek archipelago, it is really manageable. But remember the because it is so full of mountains, what looks like a short drive on the map, might be much longer in real life.
Also, steep roads means the car need a lot of gas, make sure to fill the tank before you go, because there arent too many gas stations, and you dont want to be left somewhere in the middle of nowhere without gas!
Another thing: some roads are in such bad shape that your rental car insurance doesn't apply there. Check in advance.
Mount Olympos. To anyone a little bit familiar with greek mythology, that seems like a must-see place. Meeting with the Gods, perhaps, but watch out because this is not a joyride if you have any fear of heights. The road up the mountain in really narrow and in poor condition with the steep hillsides really close. Consisting of stones you fear anytime the edges will just roll away under you. Instead, stop before you enter the white stoney area, the view is actually almost the same.