At the heart of the animated “Hotel Transylvania” movies lies an important message: Accept people for who they are, respect their differences and never judge a book by its cover. Or, as Count Dracula says in the voice of Adam Sandler: “Monster, human — what’s the difference?”

It’s a lovely thought. Why, then, is sitting through one of these movies almost as pleasant as being burned at the stake?

It didn’t have to be that way. With “Hotel Transylvania 3: Summer Vacation,” the franchise returns to the romantic-comedy formula, placing the widowed Dracula and his family — daughter Mavis (Selena Gomez), son-in-law Johnny (Andy Samberg) and grandkid Dennis (Asher Blinkoff) — on a cruise ship.

The romcom is Sandler’s sweet spot, as he’s proved with Drew Barrymore in “The Wedding Singer” and other titles; here he’s paired with Kathryn Hahn as Ericka, a chirpy cruise director who catches his eye. Unfortunately for Drac, Ericka is the great-great-granddaughter of monster hunter Abraham Van Helsing (Jim Gaffigan). Her first thought is murder — in one ploy, she laces Drac’s guacamole with a lethal dose of garlic — but in the end, love conquers all.

What prevents “Hotel Transylvania 3” from sparkling is the dull material. Robert Smigel and Sandler have bowed out as writers, replaced by three-time director Genndy Tartakovsky and Michael McCullers (“The Boss Baby”).

They generate a lot of filler: dance routines with pratfalls, a volleyball game with a living ball, the inevitable passing of gas. Meanwhile, Tartakovsky’s super-cartoonish animation is beginning to grate. There’s little rhyme or reason: Limbs go rubbery or angular at odd moments; simple faces are framed by hyper-detailed hair; Drac’s mouth alternates between 3D lips and a simple line.

In the end, the noisy humor and classic rock songs drown out whatever worthy messages “Hotel Transylvania 3” might offer.