Regarding keeping M2 above the waterline and retrieving your footage on SD card:
I'd recommend a combination of phantomrains M2 wetsuit & sea lander, as well as a
Marco Polo Tracker (water resistant version) for any over water flights. The sea-lander will catch wind, but an acceptable compromise, esp if your main concern is to get your SD card back (with the added bonus of your M2 not being water-logged and done for the rest of your trip)!
Since you'll have several new factors to deal with that your not used to, such as AC not having reliable GPS, incorrect/no RTH bearing, forced ATTI mode, heavy winds changing direction, and wet/sleaty weather, I'd rather know that if I cant fly my M2 back, that it'll have a platform to land on, so it can be found floating intact, a bit of splash on it, instead of bobbing around water-logged, or not finding it at all?
In addition, the wetsuit would also insulate your battery beyond what you'd need!
I'm a big fan of MarcoPolo because its purely RF direction finding based, so no cell or GPS reqd. It gives you a direction & a means of figuring distance via signal strength, so your not searching randomly, doing grid patterns in your boat. Over water your range would be over 1 mi easy, likely 2mi?
Great piece of mind allowing you to squeeze juice from the battery if you run into a great filming opportunity (whales, etc) on your way back, with 30% left. You know what I'm talking about.
As for filming gear recommendations, No lense filters is probably a non issue unless your going for that 'cinematic blur, 24fps' look.
Maybe check out an Anamorphic lense? I heard there is one for the
M2P, but not sure if for the
M2Z?
Good luck. Sounds like a great trip!