In life, Keilhauia probably measured approximately 4 metres (13 ft) in length; it can be distinguished by other ophthalmosaurids by the wide top end of its ilium and the relatively short ischiopubis (the fusion of the ischium and the pubis) compared to the femur.

Judging by the partially preserved holotype specimen, Keilhauia has been estimated at 3.8–4.3 metres (12–14 ft) based on comparisons to the related ophthalmosaurid Cryopterygius.

[1] It is difficult to estimate the exact number of vertebral segments that were present in front of the sacrum; the preserved centra suggest that there are at least 43.

[1] Each neural spine is about the same length as the underlying centrum, which is similar to Cryopterygius[2] but not Ophthalmosaurus (in which they are proportionally longer in the posterior cervical vertebrae).

[8] The ribs are shaped like a figure eight in cross section near their top ends, but this is less obvious closer to the bottom; such a morphology is typical among ophthalmosaurids, except for Acamptonectes[11] and Mollesaurus.

[1] The clavicle, which is not fused to other elements in the pectoral girdle, bears a thickened process on its frontal bottom edge which points towards the midline of the torso.

The acetabulum, on the lower end of the ilium, is thickened relative to the rest of the bone, and does not bear any distinct articular facets.

[1] The ischium and the pubis are fused together into a single, continuous, solid trapezoid-shaped element known as the ischiopubis, with the wider edge (1.4 times the width of the other end, which is shorter proportionally than that of Aegirosaurus,[4] Janusaurus,[15] Ophthalmosaurus, and Athabascasaurus[6]) being at the midline of the body.

This complete fusion is also seen in Janusaurus,[15] Sveltonectes,[13] Athabascasaurus,[6] Aegirosaurus,[4] Caypullisaurus,[5] and possibly Platypterygius australis;[8] meanwhile, Ophthalmosaurus, Cryopterygius,[2] Undorosaurus, and Paraophthalmosaurus retain a small hole in the ischiopubis.

[1] Four ichthyosaur specimens were prepared at the University of Oslo and subsequently described in 2016; they include PMO 222.655, the holotype of Keilhauia, discovered from the Berriasian portions of the Slottsmøya Member in 2010.

Meanwhile, the species name nui is derived from the acronym of the environmental organization Natur og Ungdom, the fiftieth anniversary of which occurred in 2017.

[1] Keilhauia hui Arthropterygius chrisorum Undorosaurus gorodischensis Platypterygius hercynicus Platypterygius australis Caypullisaurus bonapartei Brachypterygius extremus Maiaspondylus lindoei Sveltonectes insolitus Aegirosaurus leptospondylus Acamptonectes densus Leninia stellans Ophthalmosaurus icenicus Mollesaurus periallus Athabascasaurus bitumineus Cryopterygius kristiansenae Palvennia hoybergeti Keilhauia can be distinguished from its contemporaries by various characters.

[20] The seafloor, which was located about 150 metres (490 ft) below the surface, seems to have been relatively dysoxic, or oxygen-poor, although it was periodically oxygenated by clastic sediments.

[21] Despite this, near the top of the member, various diverse assemblages of invertebrates associated with cold seeps have been discovered; these include ammonites, lingulate brachiopods, bivalves, rhynchonellate brachiopods, tubeworms, belemnoids, tusk shells, sponges, crinoids, sea urchins, brittle stars, starfish, crustaceans, and gastropods, numbering 54 taxa in total.

[15] Additionally, 21 plesiosaurian specimens are also known from the site, including two belonging to the large Pliosaurus funkei, three to Colymbosaurus svalbardensis, and one each to Spitrasaurus wensaasi and S. larseni.

Many of these specimens are preserved in three dimensions and partially in articulation; this is correlated with high abundance of organic elements in the sediments they were buried in, as well as a lack of invertebrates locally.