Larousse Lexis gives 3 defs for "fontainier":
1. (Vx) Fabricant, marchand, réparateur de fontaines
2. Agent municipal chargé du service des fontaines publiques
3. (1538) Celui qui fait des sondages pour amener les eaux souterraines à la surface du sol.

None of these quite fit the Versailles context if, as you say, they "brought in the water supply" rather than exploit a natural spring at the site of the Grand Canal. But since they had to a) dig the Grand Canal and b) (presumably) dig further smaller canals to bring the water to the Grand Canal, they must have been canal builders.

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Note added at 43 mins (2005-02-27 12:25:10 GMT)
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Interestingly, Ferdinand de Lesseps, who built the Suez Canal, was born in Versailles (in 1805)!

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Note added at 53 mins (2005-02-27 12:35:07 GMT)
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Which set me thinking about \"engineer\" ... On second thoughts, I prefer \"water engineers\":

The construction of the Marly machine (an emmense hydraulic machine, driven by the current of the river--since destroyed) allowed the transport of water from the Seine River. The park developed around the Grand Canal, an artificial pond of about seventy acres with a circumference of almost five miles. The hundreds of fountains and waterfalls drew from an elaborate underground network of reservoirs and aqueducts.
[http://www.nv.cc.va.us/home/cevans/Versailles/Site/mainbuild...]

At Versailles, these forces that seemed to threaten the \"natural\" hierarchy of noble political control were hidden. The WATER ENGINEERs from Paris who built the early supplies for Versailles placed their pumps and holding tanks outside the walls of the gardens, where they would not be immediately visible to those touring the grounds
[http://legalminds.lp.findlaw.com/list/rre/msg00239.html]

When faced with a deep valley, Roman engineers should use pressurized pipes that are inverted siphons. Roman WATER ENGINEERs build these rather than arcades because tall arcades are too unstable when built too tall
[www.prek-12engineering.org/data/d39/Aqueduct.doc]

the place, with the assistance of all sorts of architects, WATER ENGINEERs, landscapers and ... It is a series of stunning gilded statues and fountains and water ...
www.bespolka.com/bespolka web site/ 03%20RussiaMongolia/Russia%202003/Russia%202003-09-03.htm

fountains to sewage systems. As experienced WATER ENGINEERs and pump ...
www.the-pda.com/Documents/ Member%20Details/Mem-Allsebrook.htm