A sea voyage would improve his vision because of the continuous
wobbling and shaking about on the ship/boat. This movement causes the
illusion in one's field that the surrounding objects are moving in an
oppositional direction. This illusion is restful to the mind and
therefore improves the sight by relieving strain which causes
imperfect sight. Please search on google for "Dr. WillIam Horatio
Bates" to find out more about this phenomenon.

I also presume that the recommendation was given because a sea voyage
would improve his general health by being in the open fresh air, and
by eating fresh seafoods, which may relieve or ameliorate his
measles.

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