Watch Steven Frisch free solo the the first pitch of Bridal Veil Falls (WI5) in Valdez, Alaska, with a sea kayak hanging from his harness in this clip from 2015. Why? That's a flawed question, it assumes that there could be a reason to do this. That said, Frisch explained in the Youtube comments, "I had arguments with people and I wanted to prove that I can free solo WI5 so well that I can even do it with the weight and awkwardness of a sea kayak. Also, any extreme challenge has an attraction to it!" It's as good a reason as any, considering that there are rarely seas above ice climbs.
Frisch did have a contingency plan, should things have gotten out of control. He explained on Mountain Project, "The line to the right is a rope I set up an hour before I climbed with the sea kayak, in case a crampon broke or I got scared and wanted to rappel off the climb. I used it to rappel down after I reached the cave. Unsurprisingly, it was pretty awkward and hard pulling the sea kayak over the lip into the cave one pitch up because there is a drop down into the cave—so the weight of the kayak pulled me in an upward direction as I tried to climb down into the cave."
This is not the only time Frisch has climbed with the kayak. He'd previously free soloed WI4 with the boat, as seen in this clip:
And perhaps to train for climbing with the vessel, he has also soloed with a 45-pound weight.