Narrowing coastlines do have an effect; the tides in the Severn Estuary are some of the highest in the world, precisely because the coast funnels the moving water as it comes inland. Basically, the tides are a wave created by the moon's gravity, and to a lesser extent the sun's. When the two coincide, we get the spring tides, when they oppose each other, we get the neaps. Wind and coastline do have an influence on the height of the resulting tide, but they don't cause it.