i think you have 2 options, making the levels by hand by placing colliders, or using the canvas plugin to check for rgb color, i would say that option one would be the fastest, option 2 is how lemmings and worms worked, checking different colors and based on this you can move or not, but its pretty difficult and will get very complex, but if you wanna try..
you could also make different shorelines pieces with collison set, and generate them randomly on y-axis, you would need pieces that would fit on each other ( with some overlap maybe) but could be nice..