SWEETHEART is, first and foremost, a monster picture. Like many monster pictures, your primary burden as a viewer is to accept a fully realized, highly evolved and niche-adapted monster for which there is no previous record with no more foundation than "This monster walks into a bar, see, and...". Once you can do that and get on with the movie, everything flows pretty well.
Here's a preview of the first 3rd or so:
SWEETHEART opens a few feet underwater along a sandy shoreline decorated with what looks like eelgrass. As our perspective bobs above water, we see someone, obviously just washed up on shore, lying just within the water's edge dressed only in shorts, a summer shirt and a life vest and who, upon closer inspection, turns out to be a girl named Jenn. Apparently she's been through a difficult experience and appears dazed, but eventually gathers her wits about her and notices another person a few feet away also lying close to the water's edge and similarly sporting a fetching lifejacket.
This turns out to be a young man in bad shape due to a good sized piece of coral sticking out of his side. He doesn't live long. Jenn drags him a few feet up the beach past the high tide mark and leaves him.
That night, Jenn experiences a nighttime storm. The next morning there are several fish washed up on shore which Jenn collects. While doing this she discovers a badly mangled small shark exhibiting a striking collection of parallel gashes. Claw marks?
Jenn does a little survey of the local area and finds some odds and ends suggesting that the small island was previously inhabited by people. She finds a Game Boy, headphones, a dirty old cooler with some badly expired food and a couple of bottles of Coca-Cola, a collection of personal effects and so on. Also on one of these little exploratory forays, Jenn discovers a little graveyard with a small collection of residents. Perhaps the owners of the items she has been finding? And where is the person or persons who did the burying and placed the grave markers?
In the hot tropical weather, the dead friend on the beach begins to get a little... perfumey. Jenn digs a hole where the beach meets the tree line and helps her dead friend settle into more permanent accommodations.
Someone, or something, however, does not seem to approve of the new arrangements. On a little foray along the beach in the morning, Jenn discovers that her previously interred friend has been messily exhumed and gone walkabout. Given the amount of bodily fluids spattered around, does this imply that the someone/something that performed the exhumation did a little snacking in the process?
That evening, Jenn hears something large moving around nearby. And not only that, the guttural, animal noises it makes would terrify a granite statue.
The next morning, always on the lookout for something she can use, Jenn sees a colorful suitcase adrift not far offshore and swims out to get it. While collecting this little treasure, she happens to look around underwater and, maybe 40 feet down on the seafloor, she sees a large and somehow ominous hole. Even though it's in the sand, it seems to have strangely vertical sides with sand that swirls in a circular pattern around the edges suggesting seawater is slowly draining into it. Jenn returns to shore with her booty.
The next night, Jenn seeks slightly more secure bed accommodations and holes up inside a hollow log, although it doesn't really seem all that secure, really. But in the night, her growly, mysterious admirer finds her and makes at least a halfhearted effort to get at her inside the log.
Jenn determines to learn a more about her terrifying island companion.
The choice of the title, SWEETHEART, is a bit unusual given the subject matter. It turns out to be the pet name for Jenn, the central character for the movie, used by her boyfriend.
All in all, SWEETHEART is an excellent monster picture and the bulk of the plot line unfolds legitimately and naturally once you accept the premise. I am an unreasonable stickler for detail and there were a few irritating points that didn't really fit or were questionable to my way of thinking, but I forgave those under the heading of poetic license.
The only actual heartburn I had with SWEETHEART was the ending. The movie clearly has meta-statements to make about Jenn's personality and utter dependency on others for her maintenance and well-being. It's a monster picture at one level and a journey from dependency to self-reliance on another. Once Jenn has arrived at a state of self-reliance, the movie comes to an end. In my view, however, there were dangling threads left in the practical, monster-picture-part of the plot line that left me with an unsettled "But what about the..." sense.
Aside from these issues, two thumbs up.
A note about my star rating of 8/10: this sort of rating may make you think that I'm calling SWEETHEART a Pulitzer prize-winning work of literature. Nope. I rate movies on a subjective, relative scale based on my perception of how well the movie achieved the realization of the movie it intended to be. I believe SWEETHEART was extremely close to the movie it intended to be. Hence the high rating.