"Hermès only to swim," Disick captioned the photo on Saturday. Hamlin responded with heart and fire emojis.
The pair kept it cozy in matching off-white sweats while lounging on a black speedboat emblazoned with "LORD," Disick's self-imposed nickname. Penelope got warm with them under the blanket before joining a friend in sightseeing over the edge of the boat, which included some dolphin and seal appearances.
Hamlin appears to have grown close to Disick's daughter after Penelope celebrated her 9th birthday earlier this month. "Little peesh," Hamlin wrote to the birthday girl in the comments of Disick's Instagram post. "Happy birthday to the best facialist in town."
Watch: Scott Disick reveals he is branching out into the beauty world
Disick also shares sons Mason Dash, 11½, and Reign Aston, 6½, with ex Kourtney Kardashian, 42, who's recently been dating Travis Barker, 45. Hamlin posed for a photo with Reign last weekend during another family outing.
The Flip It Like Disick star was first linked to Hamlin in October and they made their relationship Instagram official during Valentine's Day weekend.
A source later told PEOPLE in March that Disick "is getting serious" with the model. "It's a different type of maturity, and they really don't feel the age difference," the insider added.
Hamlin's mother Lisa Rinna made her feelings about Disick known on Watch What Happens Live in May. "[He's] very similar to what you thought when you met him. He's more handsome in person," the Real Housewives of Beverly Hills star, 58, told host Andy Cohen. "He was very nice, we had a very nice time. ... It is what it is, guys. It is what it is."