I'm all for AEDs being as common as fire extinguishers out there, but it's important to remember that the AED will only function/administer a shock when it detects ventricular fibrillation. Sure, it's good to have on hand, but most recent AHA guidelines suggest that unless you're dealing with a witnessed cardiac arrest with the defibrillator immediately and readily available (I don't think the time necessary to bring an unconscious diver to the surface qualifies), the first care to be administered is 2 minutes of good quality CPR (minimum 100 compressions/minute), 30 compressions to 2 ventilations with full chest recoil prior to application of the AED... Eisenberg criteria aside (Eisenberg: chances of successful conversion from ventricular fibrillation (VF) decreases 50% for each minute VF persists).
Yes, I agree completely if there's a AED available, bring it along, but it's not always the be-all, end-all, cure-all that folks believe it to be. In my limited experience as a professional paramedic since the mid-1980s, patients suffering from cardiac arrest after a drowning/near-drowning incident usually aren't in VF, so the AED would do nothing for them.
Make sure your O2 & First Aid kits include a pocket mask/barrier device or some form of manual resuscitator (bag-valve mask, etc) and that folks are familiar with their effective and proper use. Be up on your CPR skills and make sure your emergency plan includes the ability to summon your local Advanced Life Support Provider. If there's an AED available, ok, but just remember it might just tell you "no shock advised... check breathing... check pulse.... if necessary, begin CPR."
And please, please, PLEASE: Use the AED safely. Remove the patient from standing/pooling water (that can conduct electicity to you) and ALWAYS MAKE SURE no one is in contact with the patient BEFORE pushing the flashing orange button.