People hunt whales for meat, oil, whalebone and ambergris, and for scientific research. Whale oil is used as a lubricant and as a component of soaps and cosmetics (and was formerly used for margarine and as fuel for lamps). Ambergris is used as a fixative in perfume production. Whale hunting is a way unwealthy people make money illegaly in places like Iceland, Japan, etc.
A loose fish is a whale that showed no signs of being hunted or that escaped from a whaling crew.
Crow's nest
starbuck
http://www.eganinstitute.org/Images/Whaleboat-1.jpg
dreary
the log and line
Tashtego
These were whales who had infamous markings of sorts. They were even rumored to have destroyed ships. Though this may be apocryphal.
London
Sympathy
Henry was displaying bravery
somber