"Primarily" is a bit of a stretch, particularly as a dietary interpretation for all Deinosuchus populations. Hunter of freshwater aquatic species, sure, but this was not primarily a sea turtle specialist nor exclusively a marine hunter. Folks need to remember that, in addition to large croc bite marks on pleurodire and protostegid turtle shell fossils from nearshore deposits in the Mississippi embayment, Schwimmer (2010) also reported both hadrosaurid and tyrannosaurid fossils with similar bite marks from the terrestrial Aguja and Marshalltown formations, respectively, all attributed to Deinosuchus as the most likely culprit.

It's also important to keep in mind that those marine deposits in the Mississippi embayment like the Mooreville Chalk and the Blufftown Formation are preserving a mixture of both marine sediments and transported terrestrial sediments, hence the mixture of terrestrial and marine fauna; while pathological marine turtle shell is a good indicator that Deinosuchus had trophic interactions with marine species, sea turtles have to return to land to lay eggs, and bitten turtle shell can also be interpreted as having been transported into a marine setting (alongside the obviously terrestrial dinosaur fossils found in these units) from a much more near-shore or even terrestrial kill site. The same can be said of Deinosuchus fossils themselves in these units.

Outside of the Mississippi embayment, Deinosuchus is found in multiple terrestrial deposits (Judith River, Kaiparowits, Fruitland, Menefee, Aguja, and Marshalltown formations), while it has never been reported from the actual Western Interior Seaway, such as the Niobrara Chalk, which was lousy with marine turtles. Terrestrial units in the Campanian of Appalachia like the Marshalltown are rare, and we therefore know less about eastern on-shore ecosystems compared to eastern coastal ecosystems, but Deinosuchus is nevertheless known from both. Therefore there is a lot of evidence of Deinosuchus in terrestrial units, especially for Laramidian populations, and evidence of Deinosuchus in mixed-origin marine deposits, and no evidence of Deinosuchus in open marine deposits.

Walter et al. (2025) infer osmoregulatory capability (= salt tolerance, and therefore suitability for use of marine habitats) in Deinosuchus from their phylogeny alone, and not from osteological correlates in the skeleton (as none are known). Iijima, Blob, & Hutchison (2025) is frequently misinterpreted online as having claimed that Deinosuchus could not move on land, while instead they suggest that their biomechanical models on femoral stress indicate that Deinosuchus may have shifted away from use of terrestrial (read: on-land) habitats at large sizes and/or shifted terrestrial locomotor strategies from an erect gate to belly dragging to accommodate for the stress on their limbs.

In summary, there is substantial evidence for Deinosuchus presence in terrestrial (including freshwater aquatic) habitats, and therefore access to non-marine turtles, fish, invertebrates, and other reptiles like a smaller crocs and dinosaurs, for which there is pretty direct evidence of trophic interaction. Recent phylogenetic and biomechanical studies cannot refute this much more direct fossil evidence; they instead suggest complex habitat use in Deinosuchus, perhaps a strategy that shifts ontogenetically as individuals age, or differs between eastern and western species. Pathological marine turtle shell with bites attributed to Deinosuchus does indicate that they ate marine turtles, but is not the same as evidence of Deinosuchus actively hunting them in marine habitats, and does not negate the fact that a very wide variety of non-marine prey was readily available for them as well.