The global positioning system (GPS) has become an indispensable navigation sensor for field operations with unmanned surface vehicles (USVs) in marine environments. However, GPS may not always be available, even in open outdoor areas, because it is vulnerable to natural interference and malicious jamming attacks. Thus, an alternative navigation system is required when the use of GPS is restricted or prohibited. In such circumstances, a marine radar, which is a standard sensor in amarine vehicle including USV, can be used for localization in coastal areas. The marine radar can extract landmark features of the surrounding coastlines. These features can be utilized for relative navigation with respect to the detected coastlines. However, coastline maps based on radar signatures may be unavailable in unexplored areas, and they may be unreliable in coastal areas with high tidal elevations. In this study, the relative navigation with respect to the surrounding coastlines is performed in the framework of simultaneous localization and mapping (SLAM) for a USV operation in coastal waters. In particular, coastline features are parameterized by using B-splines for efficient map management, instead of the conventional point cloud representation. To verify and demonstrate the performance of the proposed coastal SLAM algorithm, field experiments were conducted in actual coastal environments. The results are presented and discussed in this paper.