This Thing of Darkness by Harry Thompson
My 15-year old bought this one for my birthday. It's a whopper of a book, nearly 800 pages. And every one of them brilliant.
It covers the story of the voyage of the Beagle in the 1830s. Yes, that Beagle, which carried Charles Darwin on the journey which helped him formulate his theories of evolution. It is the tale of both Darwin and Robert FitzRoy, the captain of the Beagle and a scientist in his own right. The book follows them both during the voyage and in the years afterwards, until FitzRoy's death thirty years later. It is action and adventure on the high seas, and also shows the unfolding of ideas which ultimately led to The Origin of Species.
I can't do it justice here, but if you like action, if you like science, if you like historical, you'll love this book. (If you're into fluffy romance give it a miss!)