I'm going to add a couple of points here. When we were there for 2 weeks last month, the Club O beach was a lot more crowded than usual, especially for low season - most likely because of the closure of some of the other bars on the beach. When the 4 of us got there at 10 am on several days, it was quite difficult to find chairs, and harder to find umbrellas. We managed, but it wasn't easy. Hopefully, by the time you go, the construction will be over and things will get back to normal. However, I would still advise getting there no later than 9 in high season.
As for towels on the chairs, no. Only tags (or you physically sitting there waiting for the chair guys to come by) will reserve your chair. We saw people removing towels from otherwise empty and untagged chairs when we were there. We heard one of the Club O guys telling another couple that it was okay for towels to be removed as those chairs had not yet been paid for and it was obvious no one was physically there yet. So best advice is get there early.
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