well if it helps anyone else, i decided to just start a new game and rush through all the missions to get back to where the bug occurs, i turned down ALL my graphics settings to low once i reached the sea of fates mission. IT WORKED! so im guessing something during gameplay bugs this mission. starting a new game is hugely frustrating though cause of all the unskipable cut scenes and losing progress from exploring but atleast im in england now!
took me roughly 30mins to get back to the sea of fates mission rushing through everything smashing space bar to skip what i could in cutscenes etc.

@ubi-baron
come on guys fix this! game breaking bug at the moment can't progress no mater what i try. ON PC
tried lowering graphics didn't help, it sails to the middle of the ocean and they drop the sail and just sit there afk. can only exit and reload.

i think it has something to do with the blacksmith, he is sitting in the water before we take off and starts swimming after us when we leave! surely this is the issue causing it to bug isn't he supposed to be on a boat??

im on uplay+ so this is extra frustrating as its time i can't play the game wasting my subscription.