Ocean waters contain vast quantities of minerals, but most are too widely diffused to be of economic significance. However, common salt, magnesium, and bromine are largely derived from ocean waters. The ocean beds are also rich in manganese nodules.
Source: Chapter 5, Mode of Occurrence of Minerals
The question is directly lifted from point (v) of "Mode of Occurrence of Minerals." Examiners expect you to state the limitation (too diffused for economic use) AND give the specific examples (common salt, magnesium, bromine, manganese nodules). Missing the examples will cost you marks.