There are more than a few online volumes that deal with biographies of naval figures, some more voluminous than others. But I think a first stop should be a monumental biographical tool published at the end of the 19th century and, not counting supplements, comprised 63 volumes. I am speaking of the well-known Dictionary of National Biography, covering all manner of British figures from the earliest times until the present.(Of course, one of the criteria for inclusion is the subject has to be dead. A minor quibble if we are dealing with individuals mostly associated with The Age of Sail.) Happily, the entire run of volumes is available online for perusal. As is the index, but do not be fooled, the term index here refers to a one-volume alphabetical listing of the biographies with a scant bit of information attached to each entry. You cannot search the volumes looking for first lords or admiral or captain with any hope of success; you can, however, look up an individual by name and retrieve a biography written at a high level of scholarship. Here is how you access this great reference tool: firstly, come to this wikipedia entry which at the bottom has links to the volumes in various online book repositories; secondly, select the volume you wish to explore, for example, volume 11, which contains the biography of Thomas Cochrane; thirdly, having selected that volume, you would then choose what format you wish to view the volume in; fourthly, use the appropriate “find” or “read” command to locate a authoritative(at least for the late 1800s) article on Cochrane. It is a bit cumbersome, but at least you have access to a wonderful tool that was published until fairly recently. This work was succeeded by the Oxford Dictionary of National Biography in 2004 and while this is NOT freely available online, many libraries either have an electronic version or its printed counterpart. What is available for you is the name index; if you search for Thomas Cochrane, here is what you get:
Oxford Biography Index entry
Cochrane, Thomas, tenth earl of Dundonald (1775–1860), naval officer
Oxford Dictionary of National Biography
A much fuller biography is in the actual work itself, written by the esteemed naval historian Andrew Lambert.