I believe it's on the sound-side, so swells and riptide are non-existant. Somebody mentioned Bass River, which is a place that you'll have a good chance for catching blues or schoolies. If you are going to fish west dennis beach, keep going until you can't go any further.
I'm not sure what time they close the gate (or if they do at all). Be wary of that. There are other beaches around on the cape cod bay side, that may be better than the sound-side beaches. Most are sandy, with no surf. Some have jetties, and some have low-tide where you can wade way out. Night fishing will be your best bet in august, or so they say.