Calisphere

The University of California's Calisphere describes itself as, "A world of primary sources and more." If you're looking for specific, primary resources, the search box (on the upper right of the site) returns easily navigated results under a button bar that lets you choose images, texts, or websites, with an additional "search within results" box for easy limiting. And although the site is sponsored by the University of California, it includes resources from many repositories, large and small, and the resources are not limited to those relating to California history--for example, the Grunwald Center for the Graphic Arts, UCLA Hammer Museum, has contributed images from Hiroshige's Thirty-six Views of Mt. Fuji.

A great strength of the site, though, are the collections for educators which lead to sites external to Calisphere, such as UC San Diego's California Explores the Oceans--Expeditions site, as well as Calisphere sites on topics like Richmond Shipyards.

As for maritime resources, the site contains thousands of items related to boats, ships, shipbuilding, etc. And although the site is rich, it's not comprehensive--it doesn't contain all of the resources from the contributing repositories--it's a curated collection, and a great place to start one's research.