Just got back from a week in Poros, staying at Friderika. The blue-flag area of the beach is pebbles and a bit painful for bare feet, but otherwise great: when we were there, the umbrella + 2 sun loungers was 6 euros per day, there are loads of tavernas all along the waterfront so you don't have to go far for refreshments. The sea bed does shelve down but not too steeply and if you've got a mask and snorkel, there are are quite a few different fish for you to ogle at. A chap there warned us he had been swimming near the harbour area and got nipped by some eel-like fish (it didn't break the skin or anything), but we had no adverse experiences. Also, on a clear day you can see the mainland. We liked it!
I can confirm that the home-made chocolate cake at 'The Castle' (or 'To Kastro', to show off) cafe was gorgeous and only 2 Euros! There are 2 ex-pat ladies who work there and they are both friendly and a fountain of local knowledge...oh the views from the castle are excellent too! You can see Argostoli, Mount Ainos and all the way to the east coast from there - I can recommend going at sunset!
Friderika is 10-15 minutes walk from the centre of the resort, which can become a bit tedious, but it is quiet...except for that cockerel who is still going strong! He starts before sunrise and goes on all day... we were in the lowest storey so didn't really have much of a view, but then it's literally 100 yards from the water's edge, so we were happy! Yes, there is a swimming pool 5 minutes' walk away (it's right on the seafront, you can't miss it) which is free & open to all - assuming you buy a drink or something from the bar there. Incidentally we went with Golden Sun, booked it just a few weeks before going - found it on the internet.