Convergent evolution with Mososaurs presented Hesperornis with interesting teeth for a bird. The teeth were designed to snag and hold fish, and they were good at their job, of that there is no doubt. Situated in a groove along the entire bottom jaw, except for the tip of the beak, and the back of the top jaw rather than in sockets, the teeth were sharp and conical and grooves in the top of the mouth allowed those lower teeth to be "locked" into place when the mouth was closed with just a small rotation of the jaw muscles and joints. The beak itself was evident in the remains of attachment sites and other evidence for a hard keratin shell that once laid on the outside of the nasal, predentary, dentary, frontal, premaxilla, and maxilla of Hesperornis. The nasal bone of Hesperornis, if you look at the diagram of the skull left, is squeezed up and into the middle of the foremost part of the skull, which is not something we see in dinosaurs and is worth pointing out if one is used to dinosaur skulls because it is vastly different from those.
The wings of Hesperornis and its legs and feet are a topic to be taken together for a very good reason. Clearly the wings were not capable of flight power and they were also certainly not used for diving with maybe some small slight exception in aiding with turns; even my non-aerodynamic hands can have some noticeable affect in terms of lift and drag when held outside a car going 60 down a country road. The wings of Hesperornis, though, had evolved into a mechanism perfectly suited to Hesperornis. Surely it the wings may have had some affect on the directionality of the body, but we can say for certain that they did not provide power and were not an enormous factor. If they had no influence at all surely they would have disappeared entirely in one species of Hesperornis at least. Regardless, we know that the power of the dive was in the legs, so let us move to the rear of the bird if we may and not debate the wings any more.
The legs have been written about wonderfully many times and they deserve the accolades and study. Loons and Grebes alike are powerful leg divers, but they have different adaptations which make this true. Loons have webbed feet held on legs encased in the body wall until the ankle and are very awkward on land. They can "run" a little, run is used very loosely here, as anyone that has taken bread to a pond knows, this is not the norm for all water fowl (I have been chased by geese and ducks too many times to remember). Grebes, in slight contrast, have legs much like the loon, but have a slightly different on land gait, and are a little more capable of running, and completely different feet. We imagine, because ducks, frogs, and geese are our typical land/sky/water transition animals that we come across, that webbed feet are best for swimming and that no other system comes close. However, Grebes have a different diving power generator in their feet. Their feet have large lobes of dermal material which are not webs truly, but are large spoon like adaptations on each digit which are capable of cupping and pushing water. The power of the mechanism is quite easy to see in the video here (which was also the result of a smashing good deed by the video taker). What we have are two different birds with fantastic but different locomotion. Hesperornis, though, is a bridge between the two, so we believe. Hesperornis has the gait and positioning, though the birds are both closely constructed in the leg region, of the Loon while it has the same propulsion mechanism of the Grebe. It's almost as if each exact adaptation was fantastic together but over time each one became slightly better suited to a slightly different environment and has thus presented us with Grebes AND Loons instead of a smaller, hopefully, because I would not want a 5 foot Hesperornis in my backyard, version of the ancient toothed bird.