Generally around 2 stops (ND4) between sky and ground is a good starting point for shooting into the sun that will keep everything within the DR of the sensor, but you might need something stronger if you have a particularly dark foreground like a dense forest - ND8 or even an ND16. You can then tune the balance in Photoshop/Lightroom/whatever in post to get the desired effect. I use PolarPro filters on my
M2P and they do a number of GND options that combine various starting/end densities with CPs, so you can still get to the aperture sweet spot and get some polarization - not too sure about the options for other filter vendors such as Skyreat, etc.
Note that the filter density has a fairly soft gradual transition to allow for uneven horizons, as you might expect, but you will still need to align the transition point with the horizon by angling the camera up/down, which can be done in flight. For a seascape that will be mid-frame (e.g. the camera will be horizontal), but might be slightly offset if you are shooting towards mountains. As a result, to avoid having the horizon dead-centre in the frame you'll almost certainly need to crop the image from the original - I generally shoot at 3:2, then crop to a 16:9 ratio or similar so I can place the horizon exactly where I want it for a pleasing composition.