Guneyli is peacefull and friendly bay of western Canakkale. Sea is so refreshing and it is so nice to swim among tiny fishermen boats.
There are sunbeds and umbrellas on the beach with a suitable price. You can use them if you like. Also there are some cafes and small restaurants on the seaside road. You may try some seafood and have some drinks.
Moreover if you like camping this is a good place for that. Bring your tent with you and try to stay one night in here. But Canakkale il generally wind at nights, so be careful.
However if you wanna explore the hidden beauty of Guneyli, you should continue from the right part of the bay by walking 5 min. Then you will reach an untouched bay with just couple of people. In my opinion, sea is more clear and having a sunbath in silince is much more better.
Furthermore, be sure that you have your snorkel with you because there are lots to see underwater. But if you want to take precaution for sea urchins, bring your sea shoes since the sea is rocky.