I did the same steps for both bottom and surface so I'm not sure why there is an issue. I even saved the symbology style that I used for the "bottom" data points that worked, and it doesn't work for the surface data. Any suggestions?
Typical Dissolved Oxygen levels in surface sea water are, as far as I know, up to around 8 mg/l. The way you have set up your color coding, values above 6.79 mg/l will not be shown. Just adjust the upper limit for your values or add a new class what ever makes sense in your case.
(In general, if some points does not show up, the first thing to check is if the color coding accidentially excludes some value)