A seaside is mostly used to describe the area of land near the sea. It's used in a more general sense such as "we spent the month seaside" ("we spent the month near the sea") Bank is the actual land along the edge of a river. I saw a definition describing this as the place you would put your blanket to have a picnic near the river. A shoreline is the "line" where the land meets a large body of water. This can be a lake, river, or an ocean. If you thought of a child drawing a picture, the line they drew of the water would be the "shoreline".