It reminded me of watching surfers or bodyboarders. After riding one wave, the gulls would soar up and over and fly out to catch the next one. (Flying out to the next wave is a little easier than paddling out!)
I couldn't decide which pictures to show, so here's a random assortment. You can click on the images for larger versions, and to make it easier to see the gulls when they're camouflaged against the waves.
(The gull in the photo above is close to the center of the photo. It looks quite small against a very large wave.)
If you're wondering — there were several species of gulls involved in this behavior, but most of them were Mew Gulls (Larus canus). Mew Gulls are one of the smaller gulls in this area, with wingspans of ~45 inches (114 cm):