I have been to Fu long twice now, always near the Windsurfing/Sailing Club by that bridge. You can rent beginner windsurfing equipment there. The hours are 9 am - 3 pm. They employ a lifeguard who is only responsible for their members. Technically, they pay fees for using that whole area and no one else is allowed on the beach but they don’t really care as long as no one interferes with their sailing. I saw many people out there to swim, play on the beach and build sandcastles. It’s quite a idyllic place.
Membership is actually quite affordable too, around 10,000 NT a year and it’s open the whole year round. I have to inquire more about that.
The sand is actually quite nice and smooth. The view is amazing, you almost feel like you aren’t even in Taiwan, and sometimes, you need to get away from Taipei. I met some forumosa members there the last time I was in Fu long, they were trying to surf there, but I don’t think the waves are consistent enough.