FYI, Capital One is the only card that I know of that does not charge a foreign transaction fee.
As for renting a jeep to take to the Natural Pool, the following is from my recent post in "My Aruba Story":
"We took a chance and drove to the Natural Pool, which was well worth the risk. We arrived just as a tour was leaving, so we had the place to ourselves. The snorkeling was great and the tranquility and beauty made it a highlight of the trip. On our way out of Arikok National Park, we encountered two other jeeps that couldn't quite handle the terrain."
We rented from Smart Rent a Car and our jeep did not look "pretty", but the tires were nearly new (although the spare was useless). The guy who delivered it to us basically told us that if we took it to the Natural Pool, it was at our own risk. One of the other broken down jeeps we ran into (breaks went out) was from Amigo. The other had terrible tires (not even road worthy, nevermind off-road); however, I do not know who they rented from. So if you decide to go on your own, check your tires. Personally, I loved the challenge and the opportunity to experience the area in seclusion made it worth it. A couple locals were out there renting snorkel gear for $5-$8, but we had our own. It's worth it to have at least a mask to share (the pool is small). Another lovely area that was secluded California White Sand Dunes North of Arashi Beach.
Good luck and have fun!