During the four-hour trip on a blisteringly hot day, the men smoked Cohiba cigars and downed drinks as they searched for shade to take photos of their Pateks — many on wrists, others carefully arranged on a blue tarp laid on an upper deck. They talked timepieces, quizzed one another on Patek esoterica and traded tips on how to locate rare watches. The girlfriends who were along (none of whom are watch collectors) donned bikinis and sat on beach towels, chatting, before the buffet lunch.

Mr. Kawamura rolled up the sleeves of his white linen shirt to display his blue-faced Ref. 5270G Perpetual Calendar Chronograph on his left wrist. Released in 2011, its dial was packed by Patek with the day and month at 12 o’clock, a 30-minute counter at 3, a moon phase date at 6, and a seconds indicator at 9.

It is one of the 30 to 40 watches in Mr. Kawamura’s collection, which includes a number of Pateks, but also Rolex, Piaget, Hublot, Omega and Breitling — some of which he bought and others that were passed on by his father. “Collecting is a huge part of my life,” he said, estimating the value of his watches at more than 1 million Singapore dollars ($725,210).

To show his friends, Mr. Kawamura had brought along a Patek 5170G-001 18-Karat White Gold Chronograph with Breguet numerals and a black dial, co-branded with Tiffany & Co. “For young collectors, buying is easier now,” Mr. Kawamura said, recalling that when he first started collecting at age 14, he had to pore over glossy catalogs. “Now it’s all available on the web. You see a watch on Instagram and, boom, there’s the price and its whole story — everything in a single click.”