Princess Margaret Beach (formerly known as Tony Gibbons Beach) was our favorite beach on the island. On one end you'll find Jack's Bar and a dinghy dock. The other end has a small cave of sorts and a reef for some truly incredible snorkeling. And in between is just beautiful sand and water.
The sea life was so abundant on that reef, we just couldn't believe it. Among many other things, we saw lobsters, spotted moray eels, trunkfish, lizardfish and schools of hundreds of fish.
There is virtually zero parking there. I think there are maybe 3 or 4 spots above Jack's Bar, but there's no guarantee that they'll be available. Your best bets are to A) take a taxi or B) take a water taxi from either Lower Bay or Port Elizabeth. (I believe the water taxi charged $20EC one way, which is roughly $7-8US.)
You can rent beach chairs at Jack's for $15EC each ($5-6US).