But sir, you are reading a GoT thread the day after a GoT episode. Spoilers may be available. :shrug:
Also, Euron seems to be everywhere. After smashing Yara's fleet without getting scratched he goes to King's Landing for a couple of days, then sails halfway round the world to Casterley Rock to get there basically at the same time as Greyworm's fleet. Must have had Littlefinger's Magic Horse.