Unless flying from the ME 
I'd bring 3 for sure.
ME?
How was flying from a small island over all the water with the Mavic? We're you nervous at all about it? I'll be flying from BOB so similar setup...
You're numbers are off... In perfect conditions you'll get about 27 minutes per battery... Including take off and return flight / landing.
Sorry - 54m - that's 10% difference is critical

I don't actually expect to get an hour- but I figured with two batteries that would get me a pretty decent amount without having to bring the hub and a third battery- but I guess Dan is advising otherwise...