Schiaparelli, unlike Lowell, was cautious not to declare that his canali were artificially made, but he was sympathetic to the possibility. In an article of 1893 he wrote "Their singular aspect, and their being drawn with absolute geometrical precision, as if they were the work of rule or compass, has led some to see in them the work of intelligent beings... I am very careful not to combat this supposition, which includes nothing impossible."
But Schiaparelli firmly believed that they were bodies of water despite some indications to the contrary. His preferred theory was that they were natural (and his preferred translation of "canali" into English probably would have been "channels"): "[W]e are inclined to believe them to be produced by an evolution of the planet, just as on the Earth we have the English Channel and the Channel of Mozambique." Accordingly, he named them after famous rivers: Gehon, Hiddekel, and Phison from Eden, Lethes and Nepenthes from Hades, and even some real ones, Ganges, Euphrates, and Nilus.
Schiaparelli was later similarly biased in favor of habitability of Mercury. But it is interesting that he was more cautious after originally observing the canali and the "seas" during the Mars opposition of 1877, and disclaimed that "these names may be regarded as a mere artifice... After all, we speak in a similar way of the seas of the Moon, knowing very well that they do not consist of liquid masses." Only after the subsequent opposition of 1879 when one of the bright areas darkened, which Schiaparelli interpreted as flooding, did he become convinced and wrote "It is [as] impossible to doubt their existence as that of the Rhine on the surface of the Earth." In 1888 Schiaparelli published the most detailed map of Mars to date, which included the canali, and remained in use until 1960-s.
However, Schiaparelli's late years were brutal to his signature discovery. In 1892 Pickering saw his "lakes" dissociate into clusters of dark dots, in 1903 experiments involving schoolboys demonstrated that the canali too could arise as an optical illusion, in 1907 Wallace pointed out that spectroscopic analysis detected no traces of water vapor in the Martian atmosphere, and finally in 1909 high quality photos taken at Pic du Midi observatory showed no channels. But by then the idea of Martians and their canali was already firmly entrenched in the popular culture.
P.S. Since it's HSM I'll point out off topic that Schiaparelli was also deeply interested in the history of ancient astronomy. In 1877 he published a reconstruction of the first geometric astronomical model of Eudoxian homocentric spheres based on very scant mentions in only two sources. It remains the consensus to this day.