BtP is in its element with this kind of thing ... click for more 'Latin jokes explained' (thanks, Wilfried)
Professor Ibrox explains
"Ad nauseam means 'and on to the point of nausea.' And what's this sic thing? It's short for sic erat scriptum, which means 'thus was it written'. It's normally used when quoting someone when the quote [sic] contains a mistake.
"The old pirate is using 'sic' here the way teens write 'this' on Twitter. You know the sort of thing. Someone writes 'Pineapple on pizza is a crime.' Then the next person just writes 'this' to show they agree.
"So put it all together - the captain says the Gauls are sickening. The old pirate says 'this times infinity.' The black pirate says he's feeling seasic, but knows he's made a spelling mistake so he writes (sic).
"By the way, the mysterious musical symbols that appear in the black
guy's speech bubble - I think it's supposed to be a reference to Procul
[sic] Harum's Whiter Shade of Pale, but the lyric should be 'I
was feeling kinda seasic (sic)'.
Lots more about the same song | More about Latin at BtP ... Salty Dog words ... and the band name