Short-period seismic recordings at regional and upper mantle distances from underground explosions at Novaya Zemlya demonstrate that propagation across the continental shelf under the Barents and Kara Seas appears to modify the partitioning of energy between Lg and Sn phases relative to purely continental paths in the Eurasian crust. While the underwater segments of the paths are relatively short, variations in bathymetric characteristics from path to path influence the regional wave field, with systematic behavior that can be used to establish empirical amplitude corrections for regional phases. We analyze a large set of Eurasian recordings to explore the relationship between regional phase energy partitioning and bathymetric characteristics. Maximum water depth along the path is the most influential factor for the Novaya Zemlya data. It has strong linear correlations with the logarithmic rms amplitude of Lg and the ratios Sn/Lg and P/Lg. The maximum water depth probably reflects the extent of necking of the crustal wave guide under the continental margin, which may disrupt Lg modes resulting in Lg to Sn scattering, but there is surprising sensitivity to small variations in bathymetry. Empirical relations like those found here may be useful for nuclear yield estimation and discrimination for regions such as the Korean Peninsula and Persian Gulf, where many seismic phases traverse water-covered continental shelf with poorly known crustal structure.