Bocas Water Sports (Calle 3 and Avenida A, near the ferry dock, tel. 757-9541, www.bocaswatersports.com [1]) rents single kayaks for US$3/hour, US$10/half day, and US$18/full day. Double kayaks are US$5/hour, US$20/half day, or US$35/full day.
Note: Kayaking seemed increasingly popular on my most recent visit to Bocas. I saw a number of people in the open ocean with no life jacket—not smart. A glassy sea in Bocas one moment can turn treacherous in seconds when a storm sweeps through. Do not go out on open water without a life jacket.
Links:
[1] http://www.bocaswatersports.com