Most bolides that collide with the Earth hit the sea. Limited knowledge about marine-target impacts hampers predictions about their perilousness. This study presents geological features that are particular to craters formed at sea. The features are most likely a result of the influence of the target on the cratering process. Marine-target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed. When the crater diameter is large compared to the water depth, the crater resembles its counterparts that are formed on land. Craters formed in deeper water are concentric, and often lack melt sheets and rim walls, but have deposits and radial gullies formed by the resurge of the sea. Impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater.