The travel times in Iron Man Game Mode were tuned to be on average close to the real life travel times, so the scale should be approximately 1:1.Speaking of which, I'm sailing from Bonaire to Cuba and I'm wondering how long it will take to reach it, what's the actual scale of the islands in the Ironman mode
That means you should be able to measure the distance using OpenSeaMap - The free nautical chart and divide that by the speed to get the travel time in hours.
I never tried doing that myself for PotC, but in theory it should work.
Also, once you buy a sextant and chronometer, you'll be able to see your position on the Archipelago Map.