Not sure if this has been covered but it seems fairly new.
NOAA (National Oceanic and Atmospheric Administration) has released sea floor maps and other data. Much of the side-scan stuff is available in raw format (there appears to be a public domain reader for it in Linux) They also offer to process the images you select and email you where to download the output when done. Appears that there is a lot of stuff worldwide.

Bathymetry Data Viewer |

It can take a while for the initial map page to load.