We had an enjoyable kayaking afternoon, with guide Griffin, who was entertaining and knowledgeable. But the water ended up not being suitable to enter the caves by kayak. They offered a voucher to enter land-side.
We did not have a reservation, but they were able to get our mixed party of five (60s thru teens) in, in afternoon. If you have time restrictions, I would recommend reserving ahead. Otherwise, note that you might not be able to get into your preferred time slot, as they fill quickly, but they go out at lots of different times. We looked in some of the shops while waiting. They have changing facilities, lockers and showers on site. I'd recommend water shoes to get to beach, if your feet are sensitive to the hot sidewalks/pavement. We noticed that many people left their shoes down at the beach, while kayaking.
They have single and double kayaks available. Our group used both as some had never kayaked before. They give instructions and help you through, but count on getting wet, no matter. You are going thru waves to get out into the bay (bound to be some splashing), then kayak "surfing" back in.
Remember your sunscreen!!