Might I make a suggestion?
First, forget about the ND filter to correct for exposure. It doesn't do what you think it does. For digital video, it adds a bit of blur to motion, making the video appear a bit more realistic. But this wasn't video - it was still photography.
For digital photos, ND filters are used to lower the exposure so you can either 1) add blur to motion (like waves, clouds, etc) with a longer exposure or 2) allow you to adjust the aperture to get different depth of field.
Since Mavics have a fixed aperture, the second reason for using a ND filter is moot. Since you can't change the aperture, the only thing you can change to adjust to the lower amount of light is the shutter speed. For this type of digital still, the shutter speed under normal daylight conditions makes little difference as to whether it's 1/1000 sec or 1/2000 sec or etc.
If you wanted to make the scene much darker so you could go to a shutter speed of 1/30 sec or 1/10 second, etc. then you would be aiming for an effect related to the first part (motion blur). Since you didn't mention this, it appears that you are simply adjusting for exposure by varying your ND filter (and if it was set to automatic, it won't make a difference in exposure anyway).
If you want better overall water exposure photos, you could try using a polarizing filter, but on the Mavic, you can't adjust the angle of the lens after takeoff, so you have to guess correctly before you leave the ground.
Might I suggest, for digital still photography with the Mavic to skip the ND filter entirely (unless you're wanting to get much longer exposures) and do two things - 1) shoot in RAW, so you can adjust in post to a better degree and 2) shoot in auto-exposure bracketing (AEB), with 3 or 5 steps, giving you multiple and variable exposures to choose from. You'll end up with much better results, rather than worrying about whether or not you have the correct ND filter.