Our family of three did the "Helsinki by Kayak" guided tour of about 3 hours' length. We've done a very tiny bit of kayaking before on very calm waters, but are pretty close to being beginners, so we can say that this tour is (as they say in their promo materials) perfectly suitable for those with little or no kayaking experience. Aniinna was our guide and she was fantastic, very friendly, informative, and watching out for how we were doing at all times.
The tour started on the body of water near the "Long Bridge," just in front of the Botanical Garden, went west under a railroad bridge into a bigger bay with fantastic views of downtown Helsinki buildings, then we went east into other bays, beached on a small city island for a snack (which they provided) and bathroom break, and paddled into slightly more industrial parts which was fun too. Other than when the weather got a bit rainy and windy near the end requiring us to paddle a little harder, the waters were generally super gentle and easy to glide around.
This tour gives you a wonderful perspective on the city that you can't experience in any other way. The waters in and around Helsinki are so integral to the geography and the very nature of the city that you will come to a deeper "feeling" for the city if you participate in this tour. For our family I can say we will never forget this experience. And again, we are pretty close to being kayaking rookies. Natura Viva seems like a great outfit, run by folks with a great love for their land and the desire to share that with visitors.