Well. Hmmm.
Since the USS San Pablo never existed, at least in this realm (ie there may have been a T2 based AO out there at some point named that- although I can't find her in Augie's list), accuracy to the "original" is a non issue.
As for the ship in the movie, there's abundant info including whats been noted here. The only way to build that would be from scratch. Not very hard, but it would seem to me kind of pointless.
The Panay really would not be at all similar to the ship in the story. There was an original Panay built for Spain at Cavite in the Philippines, purchased by the United States in 1899, but never in service in China.
Among her commanders were Midshipman Nimitz in 1907. obviously she was a low rung on the advancement ladder.
The Panay sunk in 1937 was not launched until 1927, long after the story here, and was built for the USN. In every way there can't be much she had in common with the ship three decades earlier in the story.
Our San Pablo was one of many ships captured after the Spanish- American war. I don't think she had much to do with the Villalobos which was 50% longer and twice as heavily armed. A very good article about those ships is here:
http://www.cityofart.net/bship/spanish_gunboats.html
No, the Pebbles was a sad little boat although with good looking hull lines, one 3 pounder and a bunch of Lewis guns.
It's yet another rich piece of naval history; the Spanish fleet in the late 19th Century. Under served as models.
I like that movie as well, but for different reasons. In an earlier time, I was a collector and enthusiast of the weapons of the U.S, Army circa 1900. The Springfield rifle, the Browning automatic rifle and the Colt ACP are all well represented in the movie. My great grandfather George Morrison, West Point 1903, served in the Philippines 1905-1913. Then Scofield Barracks, and before the Philippines Fort Apache in the Arizona Territory where my grandfather GM Jr. was born. GM spent the last twenty years of his life in the VA, and died of "infirmities from service". I always figured TB, or he fell off a horse.