There shouldn't be any particular need to do currency exchange on this trip. All three ports are familiar with American tourists and virtually all shops, restaurants and vendors gladly accept American dollars. I never found one place where I needed to use pesos - American cash or credit cards were graciously accepted and even encouraged.
Also, if you need more cash, you can withdraw them from ATMs in the ports. It will be dispensed in new pesos. My bank gave me a favorable exchange rate on the transaction and charged me their normal $2 charge for using the ATM of another bank. Then you'll want to exchange those back to dollars before you return if you have extra - you'll find that most of the ports have ample exchange huts and the ones in town will give you better rates than those in ports.
No need to change your dollars for Pesos. All merchants will be more than happy to take US currency. If fact, make sure they give you back US currency in change! You cannot exchange coin for US cash, just paper money.
Tracy, yes, they will accept dollars. I work in Mexico and assume the tourist towns are the same. By the way, when are you going? A bunch of my friends are going on the Elation on June 17th...maybe we will see you there.