Where can you download free books on marine engineering?

If you are a student registered in a college or university and you have an Athens login ID or your university is a subscriber, then you can download a number of text books on geotechnical engineering, structural engineering, surveying and engineering geology in PDF format from a website known as "Info4Education" - see the related link.

Also the US Army Corps of Engineers offers a wide range of their civil and other engineering manuals for free download.

Titles from Info4Education are available in the following fields and include:

General Civil Engineering:

Blake, L. S. (1989). Civil Engineer's Reference Book.Fourth Edition. Oxford, Butterworth-Heinemann.

Doran, D. K. (2009) Site Engineers Manual. Second Edition. London, Whittles Publishing.

Geotechnical and Engineering Geology:

Bell, F.G. (2007). Basic Environmental and Engineering Geology. Dunbeath, Whittles Publishing Limited.

Hodgkinson, A. (1989). Foundation design. Oxford, Butterworth-Heinemann.

Structural Engineering:

Cobb, F. (2009). Structural Engineer's Pocket Book, Second Edition. Amsterdam, Elsevier.

Draycott, T. and Bullmen, P. (2009). Structural Elements Design Manual - Working with Eurocodes. Second Edition. Oxford, Elsevier.

Hunt, T. (2003). Tony Hunt's Structures Notebook. Second Edition. Oxford, Architectural Press.

Surveying:

Schofield, W. and Breach, M. (2007). Engineering Surveying. Sixth Edition. Oxford, Butterworth-Heinemann.

See the related links for more information.
You probably won't get full versions of commercial books legally. However, you can get lots of material about the topic - but not necessarily in the form of complete books. Just do some searching.