If you sit in the line with your car and wait for the ferry a really hot summer day. Be smart, and have water brought with you. It is always good with cold water when the ferry is slow and many tourists want to get to Fårö.
Whatever you do, do NOT swim by the rauk fields. Yes, it looks very tempting with a white and pebbly beach and even more tempting to jump from some of the cliffs, but the beaches here consist of aeons of coral seabed development and different stages of landrise which mean that you have one shelf on a certain depth you think you can handle, and then without warning the next shelf appears like a pit, much deeper than where you've just stood. Moreover, the rauks and their position on cliffs mean that there are all sorts of odd currents and you might drift out to sea or even worse, get dragged under water in no time.