Hi Ian,
We were in Plakias 5th - 19th July last year. Like anywhere in the southern Med at that time of year it is going to be pretty darn warm. However, I am not one for too much sunbathing and no lover of really hot weather but I still managed to have the time of my life.
During the hottest part of the day we would retire to many of the tavernas along the sea front for a spot or two of ice cold refreshment and possibly a bit of lunch. This takes your mind of the heat and cools you down.
Then there is also the famous Plakias breeze. The gentle wind blowing off the sea sometimes takes the heat out of the air. A quick dip in the pool or sea and then drip drying (in about 90 seconds flat!) in the sun and breeze is a good way of cooling down.
Plakias in July is hot but not hot enough that it will spoil an excellent holiday providing you follow all the common sense precautions.
As far as water sports go, I don't think there is anything specially provided beyond what equipment and items you would take there yourself. I wouldn't worry too much about Plakias being 'sleepy'. It is what you make of it and there is plenty to do and see around the area and island as a whole. That is if you can drag yourself away from the atmosphere of relaxation and the satisfaction from the stunning scenery and amazing food and drink that Plakias has to offer.
Don’t know anything about the accommodation you mentioned but I’m sure others will be able to help.
Book the holiday and go. I'm sure you'll enjoy it.
philj