Some further thoughts on the possibility that the enemy of the "Golden Vanity" (and the "Vanity" herself) were grounded at the time of the incident.

1. It would explain why the cabin boy was in extremis after swimming back to the "Vanity". He had been swimming over dry land. You could argue, why then was he drowning ? Well, it is possible to drown in a puddle if too exhausted to stand or get up. Or, that the tide was coming in.

2. It might explain why the captain was so ungracious. He realised that some superhuman feat, for which he had promised in advance a sum of gold and his daughter's hand, was in fact a deed which anyone could have accomplished. So, as he watched his foe simply fail to float, he would no doubt have smote his forehead and cried, "Doh!". And taken measures to avoid paying out.