Calibration: Do i need to do this on a boat ? and since I have a large metal engine and electronic gear, what issues (if any) does this present ?
There should never be any need to calibrate the compass unless you rebuild or modify your Phantom.
Flying from a boat makes no difference.
Just make sure you aren't launching from on top of the engine or any electronics and you'll be fine.
Flight Mode: Do you boat launch and fly in GPS mode or ATTI ?
If the boat is moving along, it would be a good idea to start and launch in atti mode to prevent the Phantom fighting to hold position before launch.
Once it's up clear of the boat, you can change to P-GPS mode.
If the boat isn't mobile, P-GPS mode would be fine for launching.
Do you set the controller as the RTH point ? Once airborne if using "ATTI" mode do you switch to GPS mode while shooting video or stay in "ATTI" ?
Because you never want the drone to actually land on the boat, and I usually have the boat in sight so it's easy to fly back, I don't use RTH on the boat.
If the boat is traveling, you need to reset the home point to prevent the drone trying to return to the launch point when it thinks it only has enough battery for the long return trip.
Resetting home to
either the controller or the drone's location is good enough to prevent that.
Atti or P-GPS mode for flying is a personal choice.
I mostly use P-GPS and I would guess most others would too.
Use whichever suits.