Feathers can have microscopic bumps on their surface that created surface tension that repels water, But when the bird dives deep, under pressure this surface tension fails and feathers do get wet. But the preening oil on the feathers allow the water to roll right off when the bird surfaces.
Cormorants still hold their wings out to drip-dry. They look to me like they are drying off with a beach towel after a swim.
ref: : Srinivasan S, Chhatre SS, Guardado JO, Park K-C, Parker AR, Rubner MF, McKinley GH, Cohen RE. 2014 Quantification of feather structure, wettability and resistance to liquid penetration. J. R. Soc. Interface 11: 20140287. http://dx.doi.org/10.1098/rsif.2014.0287