Turn it inside out.
I tacked the 3 tube down using the cardboard tunnel, then turned the entire tube inside out and finished the seam. I found this a lot easier than trying to chase bubbles and inspect the seam looking through the tunnel. I thought I had a good seam until I looked it in the daylight and saw all the flaws.
With minimal origami folding skills, I was even able to turn the entire raft inside out through the zipper hole. After tacking the bow seam from the outside, I turned it inside out and ironed the bow 1-1 seam from the inside. (You’ll only be able to work on one side at a time, since the opposite side will be inside the other.)
This method also works well for adding attachment points. Tape them to the outside, turn the raft inside out and seal away. No AquaSeal needed.
Hope this helps,
Duncan