The fastest way to farm enough money to buy all the watches lies in the ship trade system.
Just go to your base in New Orleans, to the desk and once on the ship map, go to Port-au-Prince and either buy the 13.000 gold ship or if you already have one because you may have bought it before, make it go to Port-au-Prince. From there, buy the highest amount of spices and trade with Seville, this will net you 38.000 gold almost each 30 minutes.
If you want a steady flow, you can ease the process with a second ship, because each time you'll reach Seville, that ship will need to go back to Port-au-Prince, meaning that as one boat is returning form a trade trip, you can have another one initiate another deal and keep a steady flow of gold to your arches in order to accumulate large sums of money in order to buy the watches with ease and probably anything else you might need.