Josy, congratulations on your new Adventure! Yes, it sounds like your chain is popping in and out of your master cog as the fins pass the horizontal plane. This is normal if you are bringing your fins up that high. Since it's only happening on one side this would indicate that your fins are not completely centered. With the pedal cranks held even, hold your Drive upside down with the fins sticking up. They should be in line with the pedals and within 5 degrees +/- of each other. If not, you would loosen the cable on one side for the appropriate fin and tighten the opposite side. Flip the fins back and forth a couple of times between small adjustments to re-seat the cable and re-check. Take your time.
A perfect alignment is not necessary and not as important as proper cable tension. The popping may be annoying but is not harmful. Personally, I would not attempt to alter the cog -- I depend on it to keep my chain where it belongs.
As an alternative, you might try reducing your normal pedaling stroke slightly. Try the next pedal position forward so you will still have your full leg extension -- the shorter stroke will quicken your cadence, and the reduced knee angle gives you more power. If you're using Turbofins they should become a little easier to push. Try it and see what you think.