No, not GPO. You would not be very happy snorkeling where the Giant Pacific Octopuses reside as they are cold water animals that live at high latitudes (Northern Western US continental, Alaska, Japan, Korea) in waters that are never much warmer than 60 degrees F (and often much colder). A quick Google shows that the average water temperature in the
Marshal Islands is no lower than a comfortable swimming 80's.
The picture does not give a lot to go by and many octopuses take on sort of a generic look at any given time. That being said, your location, the time of day (ie daylight), the similarity to O. vulgaris, the red coloration and what might be a false eyespot make O. cyanea a good guess, in spite of the missing telltale white spots along the arms.
Here is a link to some images of O. cyanea in Hawaii that show a strong similarity to your picture.
YouTube and Vimeo allow for fairly large videos. If you post them there and paste the "share" link, they will be viewable directly on TONMO. On YouTube (I believe Vimeo has something similar), you can make them semi-private (ie not searchable) so that only people with a link can share/see them.
There might not be much more than an air strip there but I hope you decide to and can get SCUBA certified! The diving has got to be fantastic!!!!