Posted: 30/01/2017 at 14:48
First thing to do is clear all the gravel and liner away so it's no longer attractive to visiting cats and you can see how deep it is. Then buy, hire or borrow a Karcher pressure washer and give it all a thorough clean so you can see if it has holes or cracks.
You then have to decide about a pond or not. You don't say where you are so no idea of your winter conditions but I don't reckon anything less than 60cms deep is going to provide safe protection for any fish if you get heavy frosts. It is possible to buy metal grids across the top to stop children falling in and drowning if you decide to make it attractive to wildlife and plant suitable aquatic plants and have no fish. You could also cut your own grid to measure from mesh bought at a builders' merchants.
If you really don't want a pond, how about a gravel bed for plants that like drought. To achieve this, you don't need the base to be waterproof but you do need to fill it with good quality planting compost mixed with pea sized grit for drainage. Then plant a selection of succulents, alpines and other drought tolerant plants and top dress with gravel but, be warned, cats will love it.
The third option is to fill it with rich, moisture retentive compost such as Levington's and plant bog loving plants but make sure you put some lumps of charcoal in the base to keep it sweet.
The last option is just to empty it, clean it and fill it occasionally when you need a paddling pool for the children and then make it into a pond when they're old enough not to fall in and drown. More info on ponds, dry gardens and bog gardens on the RHS website.