Yes, but no. The collocation 'on the coastline' is correct in terms of grammar, but makes no sense in the real world. You seem to have misunderstood the meaning of the word 'coastline'. A coastline is a LINE. It traces a linear shape along the exact point at which the land meets the sea. It's the line you see on a map, as in "The west of Ireland has a rugged coastline". It's the line you look at when you see waves lapping on a beach, or a cliff edge with a steep drop towards the sea. The coastline is what environmentalists talk about when discussing erosion. Are you getting the picture? As you can see - you cannot sit and have a picnic on an actual coastline! 'Beach' or 'seashore' would be a more suitable word. I hope that helps.

yes

Perfect!

This is okay.