We have experience with kayaks and would like to go out on our own while visiting Cinque Terre. Can anyone suggest a company to rent from and what has your experience been if you have gone out on your own?
Yes, it's a popular activity and the dramatic cliffs and relatively short distance between villages including Portovenere make it rewarding. The Mediterranean can generate healthy wave activity , so due regard to safety is important as always.
There is a good descriptive account here: https://www.wildtrips.net/kayak-cinque-terre-portovenere.htm
There are several links and specific information in the section titled "Are there Boat and Kayak Rentals in the Cinque Terre?" on this useful webpage: https://www.apathtolunch.com/2018/02/every-answer-cinque-terre-travel-guide.html