Navionics is company that pioneered GPS when they brought to market the world's first electronic chart device in 1984. This site has charts, a blog, an online store, a wide variety of apps and tutorials.
Nautical Charts Online lets you view charts for U.S. waters, international waters, specialty charts and browse a wide variety of item for sale.
Fish Track offers all of the tools needed to be successful in the sport of deep sea fishing. The fishing charts tool offers the latest SST and Chlorophyll images for the world's top sport fishing locations.
The average depth of the ocean is about 12,100 feet. The deepest part of the ocean is called the Challenger Deep and is located beneath the western Pacific Ocean in the southern end of the Mariana Trench, which runs several hundred kilometers southwest of the U.S. territorial island of Guam. Challenger Deep is approximately 36,200 feet deep. It is named after the HMS Challenger, whose crew first sounded the depths of the trench in 1875.
ChartGeek has charts about oceans, space, sci-fi and many others and is a fun place to explore.