Sea bird identification is usually best undertaken with a good birding guide, such as Peterson's Field Guide to Birds, in hand. If you are able to discern the details of the bird's shape, colour and structure, you may use the information in the guide to make a reasonably positive identification of the particular bird by comparing the information within the guide to your observations.
Look at the Surroundings
Look around. It may seem strange to ask about your own location, but if you are 50 miles at sea, there are only certain birds that would be seen at that distance from shore, even among sea birds. Pelicans and gulls are near-coastal birds, while a man o' war bird--also known as the frigate bird--might be seen anywhere in the Pacific, Atlantic or Indian Oceans, as can the albatross.
By the same token, some species of sea gulls can be found hundreds of miles inland. Salt Lake City, Utah, is over 600 miles from the Pacific Ocean, but has a monument to the sea gulls who saved the crops of the city's founding settlers in 1848.
Shape of the Wings, Feet and Tail
How is the bird's wing shaped? While the albatross has a wing that is generally straight, the wings of a frigate bird are angled backwards, like a sea gull's. Is the tail short? Does it fan out like an eagle's? Is it forked? Each of these characteristics moves you closer to a firm identification of the bird.
You can compare these characteristics with those listed in a good birding guide, like Peterson's. As you note the details of wing and tail, shape of beak, even the colour of the feet--as might be the case of the Blue-Footed Booby which occupies various shores along the eastern Pacific Ocean, including Central and South America--you come closer to divining the identity of the bird.
Also, remember that not all sea birds have webbed feet: the fish eagle and the frigate bird, while sea birds, are not swimmers and would drown if they entered the water. Instead of webbed feet, their legs end in articulated claws, like those of their predatory inland cousins, with which they snatch their prey from the water without ever getting a feather wet.
Colors and Body Shape
Because birds are covered with feathers, and because feathers reflect light in different ways in differently levels of light, a bird that appears to be a royal blue in the harsh noon day sun may appear black or grey in less direct light. Note whether or not the bird you are watching has markings, like spots or a bar on its wing. Are the tips of the wing and tail feathers a different colour than the body? Such details are invaluable when making a comparison with the examples in Peterson's Field Guide to Birds.
The general shape of the body may be a clue, as well. The penguin and the sea gull have distinctly different shapes: the penguin's body is almost cylindrical, where the sea gull has a body that is more aerodynamic. The reason for this difference is, of course, that the penguin is flightless. Even so, both are sea birds.