It will be windy in the next few days. This is not a negative, but it will deprive you of the opportunity to do something fantastic.
Book a one-day cruise. You will have a fantastic time, you will go to unique hidden beaches not accessible by car, you will live a whole day-experience in the coolness of the sea!
In Sifnos you have three main options: You can rent a small boat, be the captain, and go werever you like, around the island. You can book a private cruise with your friends on an RIB boat or join one of the full-day cruises of captain Michalis and captain Fabio, and their beautiful wooden excursion boats.
Do not leave Sifnos unless you have done at least one of the three. You will have a unique experience, full of fantastic images and wonderful people.
All the boats are at the marina of Platis Gialos, but it is better to have booked earlier by phone, because the demand is high and it is difficult to find something available at the last minute. Old customers book months in advance (some at the same time they book their room)!
Contact details can be found here:https://sifnoswizard.com/listing-category/boating/