Since all of these test have associated costs, physicians attempt to appropriately order test when they believe there is a true likelihood of a given disease process or a disease process that cannot be sorted out without testing. Migraine headaches are often diagnosed by clinical presentations, history and physical exam alone.
Personally, the greatest thing you could do now is to take a CPR course offered by the American Red Cross ( https://www.redcross.org/take-a-class/cpr ) or from the American Heart Association (http://cpr.heart.org/AHAECC/CPRAndECC/FindACourse/UCM_473162_Find-A-Course.jsp ). These are excellent, state of the art courses that will provide the tools needed to provide the best care possible out-side-of-a-hospital for an individual suffering from a heart attack.
Finally, in general, until help arrives: place the person in a safe comfortable place, work on getting them to calm down and to breath comfortable and easily. Loosen any tight restrictive clothing and if they have medication for their heart that they are supposed to take (generally a small nitroglycerin pill that goes under their tongue) help them take that medication. They're likley to vomit so only a sip of water at best and otherwise nothing to eat or drink. Thats about the best you can do without CPR training, which is the next thing you should do now to increase the odds of a good outcome, if necessary, in the future.
First, get the person out of the water -
Throw them a rescue buoy, ring, float, or life vest to allow the person to remain on top of the surface. If the person is close by, use a pole, rope, etc., that could be used to pull the person to land or the side of the pool. Importantly, if you do not know how to swim AND the water is over your head, do NOT enter the water yourself as struggling drowning individuals frequently drown the rescuer who is not trained in swimming water rescue. If you are not trained in swimming water rescue, seek the closest help available. Rest assured, if your children are in lessons, they will be well protected.
Second, drowning resuscitation -
This follows CPR guidelines that can be found on line in the American Red Cross (www.redcross.org/take-a-class/cpr/cpr-training) or the American Heart Association (http://cpr.heart.org/AHAECC/CPRAndECC/FindACourse/UCM_473162_Find-A-Course.jsp).
Here is the basic CPR link from the ARC website which is well written and clearly stated. As you will see, the keys are opening the airway, generally by using a jaw thrust. Rescue breathing using mouth-to-mouth breaths, and, if needed, CPR with chest compressions (https://www.redcross.org/take-a-class/cpr/performing-cpr/cpr-steps).
In summary, 1. use the tools available on site to remove the individual from the water; 2. follow the basic CPR steps.
For you, I see the next critical step is taking a basic CPR course as outlined above.