What is the purpose of the hole in the 12 x 12 boat bow (6051)?
I didn't intend to answer my own question, but after reading through the comments and doing some more digging, I think it is clear that this was intended to be a hawsehole (hole that the anchor rope passes through).

The 12 x 12 boat bow in question is a chronological descendant of the 16 x 12 boat bow:



That larger part was created with the launch of the Pirates theme in 1989 and was used in the two flagships of that time: Caribbean Clipper (6274) and Black Seas Barracuda (6285). Carribean Clipper clearly uses that hole for the anchor rope:



The 12 x 12 bow didn't appear in a set until 1992, so my best guess is that this hole was included to match the earlier part, but set designers ended up not actually using it for that purpose.