The couple celebrated their fifth anniversary last month.
An eyewitness tells ET that Justin attended the soirée by himself "because he wanted to enjoy some time with friends."
Justin, who was dressed in flippers and wearing a snorkel, "had a great time and looked like he was in a relaxed and happy mood," according to the eyewitness.
"He was taking photos and videos and singing and dancing throughout the night, even to his own song, 'Despacito' at one point," the eyewitness says of Justin, who also posed with FINNEAS during the event. "He also encouraged other partygoers to dance and was very friendly."
The solo outing came the month after Justin and Hailey celebrated their fifth wedding anniversary.
"To the most precious, my beloved. 5 years. You have captivated my heart. I know from the depths of my soul down to my bones that this journey with you will only exceed our wildest expectations," Justin wrote to Hailey, 26, on Instagram. "So let’s keep dreaming big baby. Cheers to forever and ever. I love you with every fiber of my being."
Watch the video below for more on the couple.