Two weeks of measurements of the boundary-layer height over a small island (Christianso) in the Baltic Sea are discussed. The meteorological conditions are characterised by positive heat flux over the sea. The boundary-layer height was simulated with two models, a simple applied high-resolution (2 km x 2 km) model, and the operational numerical weather prediction model HIRLAM (grid resolution of 22.5 km x 22.5 km). For southwesterly winds it was found that a relatively large island (Bornholm) lying 20-km upwind of the measuring site influences the boundary-layer height. In this situation the high-resolution simple applied model reproduces the characteristics of the boundary-layer height over the measuring site. Richardson-number based methods using data from simulations with the HIRLAM model fail, most likely because the island and the water fetch to the measuring site are about the size of the grid resolution of the HIRLAM model and therefore poorly resolved. For northerly winds, the water fetch to the measuring site is about 100 km. Both models reproduce the characteristics of the height of the marine boundary layer. This suggests that the HIRLAM model adequately resolves a water fetch of 100 km with respect to predictions of the height of the marine boundary layer.