I read this study and the recorded figure is around 50%. They're using some probability estimation based on the 50% discovered cases to account for people who h

I read this study and the recorded figure is around 50%. They're using some probability estimation based on the 50% discovered cases to account for people who had mild symptoms but not at the time they were tested (potentially after they left the ship). They identify flaws in the study though: the passengers were mostly old, so they are more likely to be symptomatic. They also didn't test everyone on the boat, only vulnerable, or symptomatic people, so their base numbers are missing some number of asymptomatic cases. It's not a perfect study and I think you could take 19% as a low bound on the figure and in wider society 50% is plausible.