Like I said, the other thing you can do here is swim. The water is beautiful, especially between the shore and the islet. The channel between the church on the islet and the shore, is roped off to prevent boats coming in that way, so swimming in that area is safe. The water is always clear enough that you can see the bottom and (unless it's very windy) is as flat as a mill pond. Lovely. You can swim over to the island and roam around on it. There's not much to see - a chapel in a courtyard and a path to the top of the island, from which you get a nice view back to the lido. Take a waterproof camera if you want pics on/from the island.
There are only really two things to do here - sunbathe and swim. Sunbathing takes place on the lido, which is terraced and arranged into different sections. Some is down by the water, some is on more of a "garden" terrace, an some more is shaded by a glade of small trees. Beds cost 6 euro per pair, which is good value. Taverna, toilet, shower and changing facilities are available, though due to an island-wide water shortage the showers were out of action when we were there.
There's a taverna behind the Lido that does typical greek fare, including fish dishes. Adjoining that is a little garden area where you can enjoy drinks.
Favorite Dish: We didn't eat at the taverna as we never want much for lunch and were content with the spanokopita that we used to bring with us from the local bakery in Symi. We did partake of the delicious fresh orange juices though.
You can walk from Symi to Agia Marina one of two ways. You can go up and over Chorio to Pedi, and take the path around the left had side of the bay (the pictures here are from that route). Alternatively you can follow the row of disused windmills at Chorio and beyond them take up the path that leads to Agia Marina. The lower path enters Agia Marina by the boat landing at the Lido. The upper path ends up opposite the island church, at the other end of the Lido.
The views are spectacular looking across Pedi bay and down to Agia Marina. The seascape is so beautiful and the pathway, though rocky and rough, is carpeted in mountain sage & thyme. The smells are lovely. There are a lot of goats around too. We could hear them bleating, and their bells ringing, but try as we might we didn't see a single one!
I'd highly recommend the walk, at least in one direction.
Water taxi is the quickest and easiest way to get to Agia Marina. The trip takes about 20 minutes and costs 7.5 euro for the return trip. There are a few taxi boat companies at Symi Harbour, the biggest of them running as a consortium of five boats that serve all of the popular Symi bays. This is the service that we used, as it seemed the most reliable option. Other individual boats run too, but if they're full, they're full...
The guy at our hotel said "where are you going today".
"Agia marina", we replied.
"Don't sit on the front on the way back"...
It was a lovely day so we ignored the advice, and got a comfortable seat, reclining on the front of the boat. We got well and truly soaked. It was OK, quite fun in the end, because we were basically still dressed for the beach and my camera was safe in a waterproof location. We walked back to our apartment rather wet and bedraggled.
Once you're here you're here so make sure you bring enough of all that you want. For me that included a (pirate) hat and a fully UV resistant shirt for when I'd been in the sun for long enough. Also (obviously) swimming shorts (no speedos please!) and water shoes (for protection from rocks & sea urchins)
Toiletries and Medical Supplies: Plenty of high factor sun screen, insect bite remedies/repellents (though we were never bothere by insects here).
Photo Equipment: Digital camera, spare batteries. Waterproof camera or watertight box if you want to take pictues from the islet.