I suggest adding a power fractal terrain first. Then add a surfacelayer called "Coast" as a guide. Set the height constraints to between 20 and 0 meters. Then set the slope constraints to between 10 and 0 degrees. Then turn the Use test colour on and zoom way out to find an area of flat wide beach for your coastline.
Keep hitting Random seed on your terrain if nothing suitable is found near 0,0,0. Don't stray too far afield from that point or the calculations get dodgy. Instead, bring the field (or beach) you want to work with to you by hitting random seed on the terrain. It may take a while but it's better than fighting with the numbers later when altitudes and slopes don't match up.
When you find a nice smooth beach, near a mountain, near 0,0,0, then add the water, at an elevation of zero, and shut off your coastline guide. The rest is just mixing colours and textures and placing objects.
PS. It helps to set your camera to 0,2000,0 with a rotation of -90,0,0 when trying to find your beach terrain with the random button. This gives a top down view of the terrain. Once you find a nice beach set it back down near sea level.
