The current reality even with the introduction of the 911 system, is this:
Should your heart stop, chances are very good that you will expire before medical assistance arrives. Portable defibrillators and CPR knowledge are not widespread and ambulance response times are a little on the long side of the equation. Arriving at a hospital here being unable to communicate is also an impediment to the best possible outcome.
It is important that medical personnel have access to your health insurance info and any DNR instructions long before a lawyer or a far away relative can deliver paperwork. I would suggest you formalize your instructions with the aid of an attorney here, communicate your wishes with relatives at home and a couple of "close" friends here. Keep a copy of all relevant documents in an easy to get to location and tell relatives/friends where that is.
Then go shopping online for some jewelry. Get a Medic-alert fashion accessory. Have Medic-alert note your DNR, contact information for those with access to your power of attorney/DNR documents here in the DR and the contact/Policy # for your health insurance provider the DR.
If you arrive unable to speak and are still alive, chances are good that the hospital will contact Medic-alert as lots of tourists have a bracelet so it's not something completely foreign here. This way the hospital can be informed of your medical issues, wishes and be directed to your local contacts who can provide the necessary documents and instructions when decisions need to be made quickly.
Your Power of attorney person should be someone local who would be available to sign the necessary permission forms at the time they are needed so that your instructions may be carried out expeditiously to prevent undue suffering and so that you do not experience a significant delay in receiving the treatment that is appropriate.
Sorry to hear about your illness. It's good that you are thinking about these matters before you experience them. Good luck, good health with many more idyllic tropical breeze days to come.