The hair-raising moments of the creature feature “Sweetheart” come the first time our shipwrecked heroine (Kiersey Clemons) is chased and swiped-at by the clawed monster from the deep that wants to eat her.

It only comes out at night. We, and she, are only getting glimpses of this Creature from the Pacific Lagoon, Beast from 5 Fathoms.

Letting us see more would be a mistake, we think, remembering Jenn’s harrowing nights spent in a hammock she’s hung as high up the coconut palms as she can manage, or crawled into a hollow log that might resist the talons of the beast.

So naturally, that’s what co-writer/director J.D. Dillard does. His modestly gripping “Cast Away with a Creature” (Sorry, I’ll stop.) fritters away its fright and suspense as it shows us too much. It muddies its spare survivalist plot and clutters its cast with characters who might have a better bead on Jenn’s post-shipwreck sanity than she does.

If you want to know why this ostensibly taut, lean thriller didn’t get a theatrical release, even with rising star Clemons (“Hearts Beat Loud,” “Angie Tribeca”) in the lead, this is why.

Director Dillard & Co. had a promising minimalist horror pitch, but blew it in execution.

Jenn washes up on the shore of a deserted isle (in Fiji), and finds a shipmate just down the beach from her. He lives just long enough for her to shout her name into his ear, try to stop his bleeding coral reef injuring, and struggle with the Coconut Conundrum.

That temptation is the first place she goes to get him “water.” She missed the “Cast Away” narration about it being a diuretic — NOT what you need when you’re likely to die of thirst.

Jenn is a survivor. And in this script, she’s going to have help. Finding a previous shipwrecked survivor’s stash of matches, Polaroids, a water-logged diary and Coca Colas will get her started. Finding her luggage from the sailboat that sank with her, her boyfriend (Emory Cohen) and three others on board means she’ll have a change of clothes.

I love that she doesn’t talk to herself (“Cast Away” style) or to volleyball. We just observe her figuring out food, frantically working out the flare gun that doesn’t flag down a passing plane.

It’s the noises of the night that freak her out. Whatever “It” is, it devours fish — leaving half-eaten sharks on the beach. And it isn’t picky about human flesh, either — digging up the bodies that wash up on shore with Jenn.

Her shock at her predicament leaps straight into pragmatism — How do I survive it? How do I get off this island? — and assumptions. She assumes that since the damned thing only comes at night, and that means she’s safe hunting for its lair during daylight hours.

I love the set up and the setting, and Clemons makes a plucky, attractive heroine to take the ride with. But Dillard goes wrong when he washes away the monstrous mystery, and when he introduces other characters and their back-stories to the plot — late in the second act.

That makes the finale, however satisfying, inevitable in the extreme, and the terrors that set it up not the least bit terrifying.

MPAA Rating: PG-13 for creature violence, some bloody images and brief strong language

Cast: Kiersey Clemons, Emory Cohen, Hanna Mangan Lawrence

Credits: Directed by J.D. Dillard, script by J.D. Dillard, Alex Hyner and Alex Theurer. A Universal/Blumhouse release, on Netflix.

Running time: 1:22