Where they sitting on the actual beach, far out at sea, diving from the air, or in the water? Perhaps a description of what they were doing might help ..
I can think of a few possibilities although I agree Gannet sounds very likely if they were diving like terns or perched on cliffs ... did you see them with binoculars or bare eye?
http://www.gobirding.eu/Images/Seabi...%20(8)%20L.JPG http://ontfin.com/Word/wp-content/up...ill_flight.jpg http://home.lyse.net/vetterli/07.05....(Medium)_w.jpg http://kayakfishinguk.files.wordpres...2/pc070089.jpg