Without more information on his life, it would be difficult to say definitively. But certainly Stonington and the surrounding area were a hotbed of activity for all sorts of maritime trade. Shipbuilding and fishing were primary industries in nearby Mystic and Noank, and New Bedford's whaling is, of course, also well documented.

Indonesia would have been pretty far afield for an American whaler in the 1780s--the industry was decimated during the Revolution:

American whaling came to a disastrous halt during the American Revolution as British naval vessels blockaded American ports and harassed American shipping on the high seas, capturing or destroying many vessels and impressing many American sailors into His Majesty’s Naval service. American whaling ports suffered, but Nantucket in particular was strangled during the war, as whaling was the primary industry there."

American whaling really came into its own after the war of 1812. (source for the above quote/info). If you look at the American Offshore Whaling Voyages database, you'll see that most voyages during that period (when a destination was specified) was Brazil or the Atlantic.

If you haven't already visited, Mystic Seaport has a great variety of resources on shipbuilding and maritime industry in Stonington and the surrounding area, which might help give a flavor of what your ancestor did and saw growing up. I'm biased because I used to work there, but it's the first place I'd start looking for more information. You can also check out the book Mystic Built as an overview.

I'll let someone else with greater expertise discuss other possibilities (trade, etc.)

If he had died a few years later the go to answer would have been the maritime fur trade, which evolved through the late 18th and early 19th centuries into the "Golden Round" trade route. Ships would leave Boston harbor, sail around South America's Cape Horn into the Pacific and up the West coast of the Americas to the PNW and Alaska where they'd sell common trade goods and buy up fur pelts (initially seals, later beavers) during the Summer then sail to Hawaii in the fall and onward from there to Macau and the Canton port of China (Guangzhou) where outsiders were allowed to trade. There they'd sell fur pelts at enormous markups and buy up Chinese goods such as tea, porcelain, silk, etc. which they'd carry back to Boston by going the long way around (but with favorable winds) through the South China Sea, the Sunda Strait (between Java and Sumatra), across the Indian Ocean and under the Cape of Good Hope then across the Atlantic. An enormously profitable venture which created massive wealth amongst American ship captains, boat owners, and traders, including John Jacob Astor who became America's first multi-millionaire partially on just this sort of trade.

However, the timing and location doesn't quite line up, as the potential profitability of this trade wasn't well known until the mid 1780s after the reports of James Cook's trips to Hawaii and China were published in Europe and America. And the first American ship to trade with China didn't leave the US until 1784.

The other problem here is the location. A ship traveling from Guangzhou down the South China Sea toward the Sunda Strait near Jakarta is going to be completely on the wrong side of the entire island of Borneo to end up drowned off the coast of Sulawesi Tengah.

But that brings us to the other question, why would you want to be over there in the first place? And this is where you get the big "oh, yeah, duh" moment, because that is where the spice islands are. The Mulucca islands (aka the spice islands) are directly to the east of Sulawesi, and trade in spices would largely have happened from the island of Sulawesi itself. That's the most likely reason an American sailor would have ended up off the shore of Sulawesi in 1780. Without more detail it's hard to reconstruct how he actually got there, was he on an American vessel? Did he end up working on a vessel for another nation or organization (as was reasonably common in merchant sailing)? Who knows.

Anyway, that's my best guess.