The girls (who are used to outnumbering the boys and even when they don't, outvoting them) and not having supreme confidence in Papa's ability to make decisions that perhaps they themselves would not, decided that to play safe they wouldn't pack any of his swimwear. That would remove temptation and eliminate the "swimming in winter" option.
But they just don't understand that pirates aren't afraid of a bit cold water.
Or swimming naked.
They might be a teensy bit afraid of soldier crabs and those little mud pellets sticking to their feet though.