Outer plumage is close-knit to help keep the down underneath dry. There's no shelter from the elements at sea. Layers of fat, and rounded body contours help preserve heat. Webbed feet, like divers flippers, are the norm, for efficient paddling through. However, this has drawbacks on land.
Many seabirds are quite ungainly ashore.
Penguins waddle round upright, so the centre of gravity is over the feet. Some petrels have to use their wings as crutches, as they scramble for their burrows when they land at night. Hooked tips and serrated edges on the bill help hold the prey, and are worn by most fashionable pelagic birds. An expandable throat helps when you swallow fish whole - head first, so the spines don't stick in your craw. Alas, that wonderful seabird, with a built-in shopping basket doesn't occur round Stewart Island.