Propagation of electromagnetic (EM) waves from radar and communication devices at sea can be significantly affected by the low-altitude atmospheric refractive conditions. Characterizing the spatial and temporal variability of refractivity is thus crucial to many important Navy and civilian applications. Surface layer models based on Monin-Obukhov Similarity Theory (MOST), such as the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM), can be used to estimate the vertical refractive structure of the evaporation duct (ED) from measured or forecasted environmental parameters, assuming a horizontally-homogeneous stratified atmosphere. As air and sea surface temperature are two critical inputs to NAVSLaM, the rapid variation in sea surface temperature (SST) and air-sea temperature difference in the Gulf Stream region provides a relatively controlled environment for investigating the utility of NAVSLaM under inhomogeneous ducting conditions. In this paper, EM propagation in the ED over the Gulf Stream is investigated using measured data from the Coupled Air-Sea Processes and Electromagnetic-ducting Research (CASPER) East Coast experiment conducted off the coast of Duck, NC, during October-November of 2015. Measurements of the one-way propagation loss between a transmitter and receiving array as a function of the range were performed during ship crossings of the Gulf Stream. Concurrent and co-located meteorological and oceanographic measurements were also collected within the marine atmospheric boundary layer (MABL), including air temperature, SST, relative humidity, air pressure and wind speed to be used as inputs to NAVSLaM. The environmental conditions in the region of the Gulf Stream are used to model the low-altitude refractivity profiles as a function of height and range, which are input into the Advanced Propagation Model (APM) for predicting the EM propagation. Of particular focus is the effect of the rapid change in SST across the boundary between the warmer Gulf Stream and the colder surrounding ocean. EM measurements are compared with the simulated propagation loss based on the range-dependent refractivity profile predicted from NAVSLaM, as well as a best-fit Paulus-Jeske ED model. Both stable and unstable conditions encountered during the CASPER East experiment are investigated.