Many thanks to anyone who is reading this and can help.

I live in the uk but I am going diving in mexico and want to take my phantom 2 with go pro.

As i will need to re calibrate it, would I be best to do it on land before i leave, but i will be travelling 200 miles off shore, the boat is made of metal, and in the instructions it says not to calibrate it near large metal structures. The boat is roughly 70 feet long, so its a big metal structure.

Or should I calibrate it on the boat?

Obviasly I dont want fly aways out in the middle of the ocean.

The captain has said they have had people calibrate on the boat, but i dont know what models they are using, what advice would you give me.

I will obviasly wait for all my satellites to lock on properly, and will be flying it in gps and atti mode, or should i stick to one of those modes for safety, I have lost satellites before and mananged to regain control with the atti L1 switch, but i wont be able to get into the water as I am diving with great white sharks.

I am currently in san diego, but i live in the uk, excuse the united states under my avatar.

Stand on the highest spot on the boat with the Phantom over your head, recalibrate that way. After your done. check the compass position on the app against the compass on the boat. If they both read the same direction your ok..

If it were me, i would try to get a compass calibration first as Kilted stated..fact that you are on a floating and moving object 200 mi. from shore, IMHO..it would not matter if you were on land or on the boat when you achieved this if the boat is a moving target!!...just my .02 as i have never attempted this before!!

I would fly in NAZA-M regardless and have S2 in the Bottom position - IOC Home Lock, at least this way, all things working as i would hope, the Phantom would at least try to get back to you on all accounts of pulling back on the right stick.

Have more fun than the sharks!!

Check out the uses of dynamic home point. It updates the home point ever few seconds so if your moving it should help the Phantom keeping home point location with up your movement. I would do the compass calibration on land before boarding. The geometric magnetic surface over water is probably nOt going to change that much. Change in the earths magnetic patters is normally due to terrain changes.
Over the water it seems to have less of an effect although the oceans bottom does cause changes but the debt of the water seems to flatten it out some. Kind of like insulation.

Is it bad to calibrate the compass too often? It wouldn't affect anything negatively right?

I'd calibrate on land and definitely do it on the boat. It may not calibrate correctly as there will probably be a ton of metal intrusions. If that's the case, the Phantom will go flashing red telling you messed up.