To put it in perspective, the Astros are directly on pace to match the 1962 New York Mets in regards to winning percentage. Of course, the Mets of 1962 played only 160 games while the Astros expect to play 162. Based on the amount of talent both the Astros and Marlins have, there is a chance both teams could finish with a winning percentage of .300 or less. Since 1890, that has been done just 24 times. An interesting aspect of this is the fact that MLB chooses to recognize the worst records since 1900. This allows people to not acknowledge two of the worst seasons possible.
In 1890, the Pittsburgh Pirates (still in the National League) finished a 146 game season at 23-113. That was good enough for a .169 winning percentage. Not a lot of information is available about that team as it is just understood the team was not any good. A more interesting story follows the only MLB team to finish with not only more losses than the 1962 Mets, but a lower winning percentage than the 1890 Pirates. The 1898 St Louis Browns finished with a 39-111, .260 record. The owners of the Cleveland Spiders, the Robison brothers, bought the Browns before the 1899 season. The fact that the owners owned two NL teams at the same time was not outlawed until later on. So, the owners decided to improve the Browns team with the best players of the Spiders team, including Cy Young. The Browns renamed themselves the Perfectos and finished the 1899 season with an improved 84-67 record. That team is now known as the St Louis Cardinals. The stripped Spiders would finish with a 20-134 record, good enough for a .130 winning percentage!
After the season, the Spiders were among 4 NL teams contracted (the others were Washington, Louisville and Baltimore. This, of course, made way for the American League which officially started in 1901. The 1916 Philadelphia Athletics finished with a 36-117, .235 record after Connie Mack traded away all his star players from his previous World Series titles in 1910, 1911 and 1913. The 1935 Boston Braves finished with a 38-115, .248 record, the only other team since 1890 to finish with a lower winning percentage than the 1962 Mets.
After the Mets would be the 1904 Washington Senators (42-110, .276), Browns of 1898, 2003 Detroit Tigers (43-119, .265), 1952 Pittsburgh Pirates (42-112, .273) and the 1909 Senators (42-110, .278). After that, out of the final 13 teams to finish with a winning percentage of .300 or below, the Philadelphia Phillies have 6 of them. Four of those seasons happened from 1938-1942. The 1938 Phillies went 45-105, .300 while the 1939 team went 45-106, .298. While the 1940 team finished with a winning percentage over .300, the 1941 team went 43-111, .279 and the 1942 team went 42-109, .278. The Phillies were back at it in 1945, when they finished with a 46-108, .299 record. For what its worth, the 1928 Phillies finished with a 43-109, .283 record.
Among the other final 13 teams to finish with a winning percentage under .300, the St Louis Browns had two more of them (1911: 45-107, .296 and 1937: 46-108, .296). Four of the five teams were all from Boston, 2 in the NL and 2 in the AL. The Red Sox in 1926 (46-107, .300) and 1932 (43-111, .279). The NL team did it in 1909 as the Doves (45-108, .294), 1911 as the Rustlers (44-107, .291) and of course as the Braves in 1935. The Philadelphia Athletics in 1915 finished 43-109, .283.
The Astros are on a path to match or finish slightly higher or lower in winning percentage to the Mets of 1962. The Marlins are just about where the 1928 Phillies and 1915 Athletics are, assuming they stay on the same pace. With parity as it is in MLB, odds are both of these teams could make history.