First, your time in Iceland will have much bigger impact potential than the journey so think more about Leave No Trace principals for while you there (I presume you'll be hiking/camping parts of the trip). Heck, even using mineral sunscreen vs chemical is better.
Choose whichever option costs less because that carries the implication that fewer overall resources were involved per movement.
Cost to provision is a function of resources required, that's basic economics.
Realize you are a minimal delta on every commercial conveyance because they're already going there and are optimized for the journey.
Here's a breakdown of Greta's stunt, yes stunt, a few years back showing it was possibly the worst way to go as it was a small scale special trip using highly specialized, ie. resource intensive, equipment.
|Construction CO2 kg
|Lifetime Miles for vessel
|Lifetime kg co2 per mile
|lifetime share kg co2 for New York crossing
|CO2 per passenger
Construction CO2 is an estimate based on the similar industrisl economies of France and Washington. 349kg is an IATA estimate. 445,000 is an estimate based on information released by 11th Hour Racing Team for yacht construction. 4500 is flight miles from LHR-JFK, but using Plymouth to New York would look even worse for Malizia II. There's only reports of 3 additional air movements but I'm going to presume the prince flew back as well.
You can tweak this or find more accurate numbers but I doubt very much it will close the gap between 600 and 23 or 2100 and 700.
You could consider hitching a ride on a recreational sail craft that's already making the journey and have some seafarer experience. But you'd still need to consider the footprint of the maritime complex that makes that possible.