In both Legends and canon, information on the original state of Coruscant is pretty thin. Protagonists don’t tend to be professional historians, and so the focus is far more often on the urbanized current state of the planet than its original topography.
It was probably an Earth-like planet. At the very least, it was neither entirely ocean nor all desert.
There were mountains. For example, one uncovered mountain remains in Monument Plaza.
Where the planet’s only uncovered mountain peak can be glimpsed and even touched.
Star Wars: Absolutely Everything You Need to Know
There were also oceans:
Coruscant covers the entire planet surface, dwarfing all the original
natural features, including mountains and (now-dry) seas....
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The most general statement we have about Coruscant’s original appearance also confirms that the planet once had forests, animals, plants, birds, and fish:
The impression was not misleading. The days in which Coruscant could
be viewed in any sort of natural state were dead and gone. The capital
city had expanded over the centuries, building by building, until it
wrapped the entire planet. Forests, mountains, bodies of water, and
natural formations had been covered over. The atmosphere was filtered
through oxygen regulators and purified by scrubbers, and water was
gathered and stored in massive artificial aquifers. Native animals,
birds, plants, and fish could be found in the museums or the
climate-controlled indoor preserves.
The Phantom Menace
All in all, it seems that Coruscant was originally a planet very much like our own, consistent with the (perhaps now Legends) idea that it was the planet that humans originally came from.
In Legends, Coruscant was originally mostly oceans, according to Wookieepedia:
Coruscant was once a world mostly covered in oceans. However, all
natural bodies of water were drained and stored in vast caverns
beneath the city as a result of years of overpopulation.
It also had at least one volcano, suggesting that the planet had a liquid mantle.
A warrior species that inhabited Coruscant during the pre-Republic era. The epic poem Dha Werda Varda recounted the battle between the
Taungs and the Battalions of Zhell, in which a sudden volcanic
eruption rained destructive ash upon the Zhell and smothered their
The Complete Star Wars Encyclopedia, P–Z
(The Zhell were also on Coruscant, so we can be pretty sure that the battle was there).
There were also mountains:
Urbanization continued at a furious pace. Soon, no point on the surface remained free from construction, save for the peaks of the Manarai mountains.
Star Wars: The Essential Atlas
Needless to say, the presence of humans and other life on early Coruscant likely requires that the planet also have had plants to maintain a breathable atmosphere, and also makes the presence of animals pretty likely.
The overall impression is one of a fairly Earth-like world.