We visit the island once a year. Our children are already asking when we are going again. We have crossed the sandbar on many occasions and you do have to be very careful to choose a calm day, morning is usually great and if you return early afternoon it's fine too. I wouldn't try to cross when the sea is choppy. I know two people have died few years ago while crossing but they couldn't swim at all and this is sea even when you can walk the water is still quite deep.
You can take the ferry of course which we do sometimes if we are taking an esky with food as there is no food on the island at all (there are some drink fountains there). If you buy a ticket for the ferry this gives you a free entry to the Penguin Enclosure on the island. If you don't have a ticket you can still buy an entry one on the island. You can also spot the penguins around the island if you go for a walk which I would recommend unless it is 40 degrees.
The water is quite warm close to the island and very shallow at the two beaches overlooking Rockingham.
There are toilets at the island but unless you are desperate I would give them a miss as they are ecological friendly ones.
Overall one of the best spots to visit if you find yourself in Perth.
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