cinque terra - time to visit

1. It looks like sand in the pictures, but it's actually very small pebbles (the size of peppercorns) and small rocks. In the other towns it's rocks and cliffs. You can be barefooted, but in my experience going to Monterosso every year when I was young, I found wearing aqua shoes better. Like most Italians, I'm never walking barefooted, not even inside the house, therefore my feet have never developed the thick bottom below, so even the smallest rocks hurt. Good Aqua shoes would be also good for hiking the trails (and don't take as much room in the suitcase as hiking shoes). Down from the trails there are great spots for swimming, if you want, but those spots have cliffs where Aqua shoes are more appropriate. Be aware that even in Monterosso the water gets very deep within 10 feet or less from the shore, therefore be careful.
2. Three nights are good.

The larger stones have a way of piling up along the tide line, so you have to go through when entering and exiting the water.

ted,

On your second question, three nights / four days will be about perfect in the Cinque Terre. Of course you'll only have a few hours on your arrival day, so probably won't be able to do much except familiarize yourself with the town and have a nice dinner.

Getting between villages by "walking" is not quite accurate. The only two Sentiero Azzurro trails that are open are more of a robust hike, at least at the beginning as there's quite a climb from Monterosso. Note that a park pass will be required to hike those.

3 nights should be fine. The train between each town is approximately 5 minutes each town. The ferry runs depending on the weather. Cinque Terre is great for hanging out at night when the tourists leave!

The underwater rocks are not a problem but I found the pebbles on the beach very painful (I'm from Australia and used to lovely soft sand). The water is great once you are in. I think 3 nights is perfect. If you can make sure you do the ferry, it's really nice seeing the towns from the water

I was barefooted on the beaches of Monterosso, no problem. Three nights four days is just right.

I wish that I had water shoes when I was there but only had flip flops,they didn't work.
Mike