My world is covered in a black stonelike material which is similar in structure to coral, being that it's highly-porous but not formed in the same way. The (non)water on my world, which is (currently) trisilane, can flow through the material with ease, and I have the idea that it flows underneath the surface to a point in which there is a different material that is not as porous, and therefore creates the true "bottom" of the ocean. This idea is not fully-formed.
You could also compare it to volcanic rock.
Islands are low-lying raised instances of this material in which these plants and their fellow living creatures would survive.
The way I define plants in this question would be "living organisms which form roots in the ground and sprout upward". They are intelligent and feeling, and can move flexibly (though slowly) to avoid predation or unnecessary touching. That is, if they can even survive in this environment.