I'm getting in on this discussion a little late, so perhaps someone else
has already made the point that I'm about to contribute.
Clemens was a good friend of John Ross Browne, who published _Etchings of a
Whaling Cruise_ in 1846. Melville reviewed Browne's book in 1851 and
actually twice mentioned Browne by name in _Moby Dick_ (chapters 32 & 56).
Clemens saw Browne frequently while he was living in San Francisco during
the mid-1860s and later again on the East Coast. In view of Clemens's
association with Browne, it would seem impossible for him not to have known
about _Moby Dick_, at least.
A person who might know more about this matter is Tom Tenney, whose
dissertation examines Browne's travel writings closely.