Gaffes

(at around 1h 45 mins) When Ariel jumps into the water and swims to go back to the castle to stop Eric and Vanessa's engagement party, she should be soaking wet, however, she is completely dry.

The long boats suddenly appears in the water.

The official poster shows an arapaima and a geophagus among the sea animals surrounding Ariel on the right, even though in real life both are exclusively freshwater fish, with the former only living in the Amazon rainforest. Arapaimas are also quite larger than shown in the poster, being the largest river fish in the world. Also shown are bettas, which are freshwater fish.

(at around 11 mins) When Ariel shows Scuttle the objects she collected from the ship graveyard, and though gannets are able to swim underwater to catch fish, Scuttle is shown staying underwater far longer than gannets have been observed to hold their breath (roughly 30 seconds compared to the minutes-long scene in the movie).

(at around 1h 45 mins) Flotsam and Jetsam both resemble moray eels in terms of appearance (and the fact that they're in saltwater), but in the climax are shown to have electric capabilities, a quality no moray eels (or technically any true eels) possess.

(at around 24 mins) The storm comes in too suddenly.

In the movie King Triton and Ursula are siblings, but King Triton is a merman and Ursula is part-octopus. They both are different species, and this is never explained.

However, it is already known from classical Greek mythology that their parents are the sea god Poseidon & sea goddess Amphitrite, whose other children included sea-horses, dolphins, seals, cyclops, and various sea monsters.

(at around 14 mins) When Ursula talks about being banished from the kingdom 15 years ago, would that mean Ariel is 15 years old? Eric is 21 years old, making Ariel and Eric's relationship have a 6-year age gap.

This isn't a goof. Nothing indicates that Ariel was born immediately after Ursula was banished. Though Ariel's age isn't mentioned in this movie, in the original she is 16.

Near the end, on the left side of the screen, an extra can be seen to slip.

(at around 25 mins) During the storm, the sailor at the helm does not make much effort to hold on to it when he is swept by the tiniest wave.

(at around 21 mins) When watching the fireworks, a small wave splashes Ariel's face and she spits out the water. A mermaid would usually have water in her mouth under water, so she shouldn't have needed to do that.

Sounds under water should be a lot more muffled, but they sound just as clear as those in air.

The mermaids' fins/tails are way too thin to propel them properly.

Steel pan drums from Trinidad and Tobago are seen being played, but had not been invented in the colonial era of wind-powered sailing ships.

It is hinted through dialogue that Ariel and her sisters all have the same mother, even though each sister is of a different ethnicity. This is never explained.

Various characters (along with most of popular culture since the 15th century) confuse two mythical beings - relatively harmless mermaids who are half-woman/half-fish, as opposed to dangerous sirens who are half-woman/half-bird, and lure men to their deaths with their irresistible singing voices.

(at around 30 mins) When the search party is looking for Eric, some footmen hold lighted torches - in broad daylight.

Daveed Diggs (Sebastian) slips from his Jamaican accent to his natural American accent in certain scenes.

(at around 31 mins) No one spots Ariel when the footmen go down the hill to rescue Eric on the beach; they should have had a clear view of a person in the water.

(at around 27 mins) When Ariel dives down to swim under the ship in search for Eric after he falls off the deck, she appears to take a deep breath before diving under the water. As a mermaid, she shouldn't need to take a breath before diving underwater.