A thesaurus is a vocabulary, usually applied to a special subject area. Thesauri can be alphabetical, or hierarchical, which shows the relationships among terms or keywords, and phrases. It can lead one to broader and narrower terms -- more general categories or specific jargon. These keywords and phrases are usually employed by indexers or authors to describe the content of papers. These terms are additional to, although frequently duplicative of, words or phrases in the titles and abstracts. Sometimes thesauri have scope notes, defining just how and when the term is employed.
Here is the link in Engineering Village (Compendex) to explore subject headings under water -- or whatever!
Engineering Village is now allowing the addition of tags to articles. They can be added in 3 different groupings: public, institutional, or private/groups. These are also then searchable.
In the GeoRef database, the Geosciences thesaurus tab is rather hidden under the tab "Search Tools". You can type in a keyword, then choose to see an alphabetical listing, a hierarchical or a rotated listing, or all 3. A rotated index shows you every every different phrase or occurrence of the term.
Several databases mounted by CSA (Cambridge Scientific) have thesauri. It's recommended to look at one or more, if they cover relevant topics/databases.
Look under the "Search Tools" tab, then "Thesaurus".
Aerospace & High Technology Database (uses NASA Thesaurus)
Environmental Sciences and Pollution Management (a direct link to thesaurus tab)
Oceanic Abstracts (same as ASFA thesaurus)
NTIS (National Technical Information Services)
GeoRef (mentioned in a separate box)