I've been in rocky seas. And I don't get seasick. Interestingly on an excursion on QM2, which of course is the Queen for TA crossings, going to the Barrier reef in QLD the waves weren't high but the small vessel was pitched around like in a washing machine. I loved it, but most were very ill including most of the crew, especially the young entertainment folk, who were extremely unwell. Even the chief engineer (whom we'd had dinner with the night before and had obviously completed many TA's) was unwell. So it isn't about heavy swell so much as how the waves and swells are throwing the vessel around. On the way back it was perfectly fine.
For world cruisers, don't go out to a Barrier Reef cruise if it's windy or choppy. And if you do, expect to be unwell for an hour or so and then be perfectly fine.