Can I claim a deduction for things that I buy to offer to customers? (e.g. water, mints)

Yes. The cost of goods that you offer to pay customers are an incidental cost of carrying on your business.

It is important that these are incurred in relation to your business (i.e. that are for paying customers) otherwise they may be deemed to be for a private use.

Please note however that water does not include GST so you would be unable to claim this on your BAS.