Colorimetric test to measure dissolved oxygen in salt water
Unfortunatly I don't think there is an easy colorimetric technique available.  

There are reagents that are strongly colored and oxygen-sensitive.  One exampleis reduced methylviologen radical.  That species can be prepared by mixing methylviologen with sodium dithionite in aqueous solution.  The one-electron reduced species is intensely blue.  It is rapidly oxidized by oxygen, leading to a loss in color.  In theory, you could use the degree of this color loss to assay DO.

The problem in real life with applying this reagent to DO determination is that oxygen in the air will also react, very quickly.  So you would have to put your seawater sample under an inert atmosphere (and also prepare the reduced MV radical solution under an inert atmosphere).  That is very inconvenient, and could create problems where your seawater sample outgases oxygen into the anaerobic environment before or during measurement.