As I'm a big fan of seascapes and coastal-scenes, he's a favorite of mine as well. I think of him as a 19th century painter, but I guess he did live for the first few years of the 20th Century. IMHO, the 19th century is the high water mark for American painters. Durand, Peale, Fitz Lane, Leutze (sp?), just to name a few. It's really an incredibly rich era for American art.
I too initially was drawn to his work because of the nautical themed works as I worked and played in and on the oceans of the world. However, some of his Civil War work for Harper’s Weekly is also good.
There are plenty of fine 20th c. artists - George Bellows ("Stag at Sharkey's"), Rockwell Kent (a d***ed commie but he was good), Reginald Marsh (a favorite of mine - Hopper with spirit and affection), Harvey Dunn (another favorite - a regional Dakota artist), Grant Wood of course, Thomas Hart Benton. And N.C. Wyeth, especially for his easel work. Like Rockwell, he got pigeonholed as an "illustrator" but had serious talent. Andrew is more popular now but he's not up to N.C.