I don't believe there are any track closures over summer, but there may be park closures on bad days.
It was a 21st December when we did the loop walk from Tapanappa to Trig via the waterfall and return via the cove. 11.2 km in a relaxed 4.5 hours, not including a break.
But it also happened to be a really wet and cold day that day.
Another walk would be down to Aaron Creek - probably starting from Cobbler Hill park - which is also the road into Blowhole Beach
With a 4WD, it would be easy to drive down to Blowhole Beach.
That is about the only spot I would swim down there, as the rest of the park coastline is very rocky, and often with very strong undertow and big waves.
It looks to be a reasonably sheltered spot - and it is a beautiful little beach.

- Descending to Blowhole Beach along the Heysen Trail. The 4WD road comes almost down to the beach on the LHS of the photo.
While the lagoon at Deep Creek mouth can be quite large with the tide in, it is a very shallow pool. ok for feet dabbling.
The "beach" is a jumble of boulders exposed to some big seas - not so good.
Nice spot to visit though.
A longer return walk could be had to Tunkalilla Beach from Tapanappa. This is a glorious 5km beach which is outside the park boundary and subject to some big surf and rips, but you should find a few places to put in the feet.
We took a bit over 2 hours to walk one way from the western end to the car park at Tapanappa.

- Western end of Tunkalilla Beach
There is a waterhole at the falls, but I don't know how good it would be for swimming.
It actually rained the day we visited in late Dec 2013, but with low flow, it looks like it can get a bit slimy.

- A view of the falls with about half the pool at the base.
Whatever you do, take sufficient water with you - both in the car and when you are walking. It can get very hot, and there won't be any reliable sources around.
Also be very aware of the fire warnings. The park has suffered some major fires at times - and there is really no where to go if you were caught in one.
I suspect they may even close the park on extreme fire danger days.
Have a great time - and I hope you get to see some whales.