The local currency is the Maldivian rufiyaa MVR, Rf, divided into 100 laari. However, by law resorts price services in US dollars and require payment in hard currency or credit card, so there's absolutely no need to change money if you're going to spend all your time at the resorts. Most hotels have a shop but this is limited to diving and holiday essentials sun cream, sarongs, disposable cameras, etc. Some excursions from resorts will take you to local islands where there are handicraft type things to buy, but they are typically made outside the Maldives and sold at outrageous markups.
If you are heading to Male or the other inhabited atolls, exchanging some rufiyaa will come in handy. The coins, in particular, are quite attractive and make an interesting souvenir in themselves, but the smaller denominations are rarely used or seen. The official exchange rate to the US dollar is floated but practically 15:1, but while dollars are near-universally accepted, shops usually exchange them at 15:1 or even 10:1.
If you want to get a local SIM card, there is a Dhiraagu shop the primary local telecom company just to the left of the airport arrivals area upon exiting. A local cell number is needed to purchase time at many wifi spots around the country sometimes reachable from where liveaboards anchor for the night.
costs
There's no way around it: the Maldives is expensive, and there is limited budget accommodation or transport. Resorts have a monopoly on services for their guests and charge accordingly: for mid-range resorts, $1000 per week per couple is a conservative budget for meals, drinks and excursions, above and beyond the cost of flights and accommodation. Practically anything — including hotel rooms if booked locally — gets slapped with an arbitrary 10% "service charge", but tips are expected on top.
There are a few Inns and B&B on the more populated atolls for the budget minded travelers.