Rather like the prior book, The Surgeon's Mate is very closely connected with its predecessor in the Aubrey-Maturin series, in this case The Fortune of War. It opens quite soon after that book, and part of the plot springs from managing the aftermath of their doings in America. In structure, I think of it as something like a criss-crossing two-parter. The book's first half covers non-naval doings in Canada and Europe, and strands from it cross over into the second half, a mission in the Balitc Sea.
(This is the book where Jack and Stephen see Elsinore and Jack reminisces about being one of the Ophelias. I giggled quite immoderately, in-between reminding myself to look at a map of the Baltic (which I never actually got around to).)
I did not find this book quite as striking as The Fortune of War, but it has its fair share of good moments. I do like the way O'Brian develops consequences in and across books; secondary characters act as it's in their character to act, even (or especially) if that creates useful plot.