Don’t mess with it!
Talk to your local REI shop and have them call their Wilderness Systems/Confluence rep. (Wilderness Systems is owned by Confluence Outdoor who also own Dagger, Perception, Mad River Canoe, Harmony and a few more companies .)
I had this happen to me a few years ago with my wife’s thermoformed Ultralight Pungo. The 3"-4" split in her boat occurred down one of the chines in the middle of the kayak close to the seat. At that point we had owned the boat for about 15 months and although we took good care of it we had used it dozens of times. This is what it looked like from inside the boat…
And I don’t think that you and I are the only ones that had this happen to them. Wilderness actually stopped making Ultralights for a couple of years while - according to my local shop - they looked for a better supplier of the ABS sheets they form these boats from. The experience put me off thermoformed construction.
REI probably isn’t going to do anything until they talk to Confluence, and Confluence will want you to do this through REI as your local dealer. We were offered a new replacement boat or a store credit for the full value.