To unlock the Playing With the Boys trophy in Cities: Skylines, you will need to to have a total of 1,000 squares of tourism specialized commercial zone. You don't need to be playing on a map that includes lots of beach/coast.
Simply setting the squares as tourism specialization isn't enough, the squares need to be built upon, so this is something that will come later in the game, even though the tourism specialization unlocks once you reach Busy Town level at around 1,200-5,000 population.
This will be easiest if you have a large city, make a separate save of it, and then draw one huge district across the whole map and set it as tourism specialization. You should begin to see the commercial zone buildings being demolished and replaced quite quickly as there will be lost of demand from your high resident count.
Fast forward time a while until the trophy unlocks.