Interesting analytical chemistry problem. Currently, it is not possible to accurately determine pH in a remote fashion. Spectroscopy is the technique of choice for remote sensing, but the H$^+$ is not that spectroscopy friendly i.e., it is a quiet ion. It will respond to radiofrequency in a powerful magnetic field but the sensitivity is quite poor in an NMR experiment by current standards. You can Google "measurement of pH by NMR". In general what appears to be pure to an organic chemist by NMR experiment, analytical chemists call it a dirty sample because there a lot of minor impurities.
A quick literature search shows that that satellite data can be used to make an only an educated guess about the ocean pH. It not accurate but good enough for a ball-park estimate.
Remote sensing of ocean pH by R. Sabia, D. Fernández-Prieto, J. Shutler, C. Donlon, P. Land and N. Reul, "Remote sensing of surface ocean pH exploiting sea surface salinity satellite observations," 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, 2015, pp. 106-109.
Within this context, the purpose of the ESA “Pathfinders-OA” project is to quantitatively and routinely estimate surface ocean pH by means of satellite observations in several ocean regions. Satellite Ocean Colour, Sea Surface Temperature and Sea Surface Salinity data (with an emphasis on the latter) will be exploited. A proper merging of these different datasets will allow to compute at least two independent proxies among the seawater carbonate system parameters and therefore obtain the best educated guess of the surface ocean pH.