If you're visiting by boat sit on the upper deck on the port side on the way out. If you can't manage that (because space is limited) use the middle deck. That allows you access to the outside. Avoid the enclosed lower deck if you can.
When you reach the tourist buses on Robben Island try to get seats on the right. All the best photo opportunities lie to your right (though you will be taking them through an open window so luck is involved).
It's an oddly attractive place given its ugly history and well worth the effort involved in getting there.
Book over the Internet if possible and before you commit, check the weather forecast. This part of South Africa can lay claim to be breezy, much like certain Scottish islands made famous by the pithy remarks of Sir Billy Connolly.
Parting thought: don't take guns, knives or wine. You go through airport-style security and they frown on such things.
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