I'd like to preface this point, this rating is based on the experience prior to the sail. I've never dealt with a company that was so clueless about how to confirm a reservation. I booked my reservation the Monday prior to the event. It said that it could take up to 48 hours to confirm the reservation. I called on Thursday and the reservation was still pending. They said they were calling the company but didn't hear anything. If they did a simple search on the company site, then they would've realized that they weren't open during the time they called. I called the WIndy City Sails myself and was able to confirm what should've been confirmed days before.
Viator was very defensive about their policies. When I read what their confirmation policy was, they were dumbfounded.
My recommendation is to go directly through the company that Viator works with since it'll be cheaper, their customer service is better, and it's less of a hassle going through a third party that seems to new to this business.