Hi David – When I first visited Ecuador, I rented a car and brought an IDP with me. They never asked to see it. As long as you have a valid US drivers license, insurance, and you are here on a tourist visa (your passport), you should have no trouble. That said, it wouldn’t hurt to contact your rental agent in advance to double-check – remember, some agencies will offer a discount for IDP.
One other thing you should know, is that when you rent in Ecuador it is common for the agency to put a hold of $5,000 on your credit card until you return the vehicle. I have rented from the Avis counter at the Manta airport, and they did this as well.
Rita and I have explored the coast using regional buses, taxis, private drivers, and rental cars. We preferred using a rental car, just because of our style of travel – we like to stop anywhere something looks interesting, and may change our plans once we are underway.
Enjoy, and let us know if you have any other coastal questions. We’ve lived in Salinas almost three years, and have explored from Playas to Canoa rather extensively.
-jim