Oddly enough, I posted information on trips to Formentera on another thread - "Blue sky in June" - only this afternoon.
As per the previous reply, I would suggest taking a pack lunch if you are heading for the beach. There are only two bar/restaurants out there and can be very busy, not to mention quite expensive, in the summer months.
Getting off at the rock and crossing to the beach is not always an option. It is if you take a ferry from Santa Eulalia or Cala Llonga, but if you take one of the large ferries from Ibiza Town, it will take you directly into the main marina. From there you can either explore the island, which is larger than most imagine and has a few decent resorts, or you can take a short taxi ride out to the beaches.
The most famous beach, and my personal favourite, is the one at Ses Illetes. It is a long, clean beach with safe swimming in crystal clear waters. As has already been said, it does have nudists dotted around, but they account for less than 10% of the beach crowd on a normal day, and tend to stick pretty much to one stretch.
The Sta Eulalia & Cala Llonga ferries will also offer you the option of going on to the marina, if you don't want to get off at Ses Illetes. As for crossing from the rock, where the ferry drops you, to the beach, the sand gets built up every now and then, so the water you have to wade across will not come past your knees and is flat calm.
I go across from Ibiza every year and will be doing so again this year. As I said on the other thread, I love Ses Illetes beach so much that I'm thinking of staying the night on Formentera this year so I can have two days there.
Hope this helps.
JJ