Would suggest a snorkel, mask & flippers. I got some good ones cheap from Decathlon...

Also a car lighter plug in FM transmitter/recharger for your ipod

If you call in to Poros (highly likely) I can recommend a restaurant up the back streets - a family owned butcher shop with a restaurant across a small road - great steaks - cant remember name but approximate location is 37.500120, 23.455295 (google map it) - cross over road from marina(pontoon) go back two streets turn right and wind up the hill - there are signs for it - I think Dmitris or similar...