The main headache for tourists in Rimini is petty theft, which is particularly common on the beach. Accordingly, you should not leave valuables unattended when going for a swim and, better still, use the hotel safes to store large sums of money, jewellery and expensive equipment.
If you are planning to rent a bike or are bringing your own, don't leave your 'steed' in the car park for too long, even if you have chained it up.