No vehicles will be allowed to drive in seawater at the beach. The company hired will inspect the vehicle when you come back, and if there are signs of driving under the sea, they will know that you have done. If you have driven in seawater, they will slug you with any fees that they can think of. It is mainly enforced on Fraser Island.
Do not drive in seawater if you want to return your bond. If you want to rent your caravan then you can explore various online sources.
Expense One Way
If you want to complete your campervan hire at a different location from where you came from (i.e. returns Vehicle different to the depot of the pickup), there is likely to be a one-way fee will apply. If you find a company does not charge one way, then you've got a good deal and you have to accept it. For example, Sydney to Cairns, or vice versa, is the most popular one by the way.
What Should Come With Campervan?
Almost all campervans for hire will come with 2 batteries "home" to run the lights, refrigerators and other 12v devices so you will not drain the vehicle's engine battery. This may be explained to you when you take your vehicle, but if the campervan you do not come with a battery of "home", but running light vans and other things on the battery, then you have to refuse to take it out of the vehicle.
It will only take a few hours of running these things to possibly flatten the battery and you could be stranded in the middle of nowhere.