I think this requires assuming H is a random oracle, no?

Suppose H(s||k) is a collision-resistant hash function. Let's build another CRHF from H as follows: H'(s || k) = H(s || k) || last-bit-of-k.

Now let's instantiate the cryptosystem you proposed with H'. Suppose m is message and that it is of length equal to the length of the bitstring produced by H'.

We have C = H'(s||k) XOR m.

The ciphertext is thus: (k, C).

What can an attacker do? Well, the attacker can XOR the last bit of C with the last bit of k and get the last bit of m. This is already enough to violate semantic security.