I am currently staying in Diani Beach.
Foil: you will have to bring your own equipment.
For Foil it is good to have a look at tides at this page: http://wisuki.com/tide/45/diani-beach
In the lowest tide it will be risky to ride here. I would suggest to make a gap 1/2 hour before/ after the lowest moment (of course the water level depends also on the moon period- if you will come in the new or full moon you will find bigger differences).
Better to come at the last days of august. Oficially in September starts the rainy season, you will find less people and most probably less wind, but every year is a bit different up here.
At Diani beach you will find lot's of resorts, hotels, few restaurants and pubs for evening hang outs. I am staying at Kitemotion school at Bidi Badu Resort. We do hang out all togather
beers, movies, pubs.
Watamu is a small villiage, where are just few hotels. So if you are looking for a kitespot with kite community attached – in my opinion better choice will be to stay at Diani- beach.
Place we stay: https://kitemotion.pl/en/
If you need any more updates just write to me on facebook: https://www.facebook.com/maria.sznajder.7