kty40809 gave a good answer. Just for your information, 맞은편 gives feelings that the said thing should be near from the standard point. Which is why we can use '바다 건너편' naturally, while '바다 맞은편' rather not. But still if you can see the island with your bare eyes, then it would be fine to say like "그 해변 남쪽으로 맞은편에 작은 섬이 놓여있다."