Halisaurus was a mosasaur. But it was much smaller than its giant relatives like Hainosaurus. It loitered in submarine caves and and cracks. It may have waited around ledges above the water where hesperornis gathered.
When the hesperornis leave their rocky ledges to dive for fish, the halisaurus are down below, waiting for an opportunity to ambush them.
Mosasaur teeth are good at piercing the skin of their prey but bad at slicing flesh. So once halisaurus has caught its prey, it swallows it. Its jaw has flexible joints within it and can open incredibly wide.
Like other mosasaurs, Halisaurus has extra teeth called pterygoid teeth, that it uses to hold on to its prey while its jaw moves forward to swallow the hapless victim whole.