The currency in Bahrain is the Bahraini dinar (BD), which is divided into 1000 fils. One dinar is worth nearly three US dollars (US$2.66, to be precise, as the exchange rate is fixed), making this one of the world’s strongest currencies, and this can get some getting used to: that seemingly cheap ten-dinar taxi ride is in fact almost $27 and thus an extortionate rip-off!
The dinar is pegged to the Saudi riyal at 1:10, and riyals are accepted almost everywhere at that rate, although odds are you’ll get your change in dinars and hotels may try to screw you out of a few percent. If coming in from Saudi, there’s no reason to change your money, but do try to get rid of any excess dinars before you leave the country, as they’re hard to exchange elsewhere, even in Saudi.