Whether you can get into the cave depends on three things: the tide, the waves, and the time of year.
It's only reachable around low tide, and even then big swell or high surf can fill the entrance and make it dangerous. Some seasons the sand level shifts and changes access entirely.
When it's open, the window is usually short — aim to arrive right at the listed opening time and keep an eye on the incoming water. Never turn your back on the ocean.