I have a really nice boat as well as a RV however there is no way I would ever consider flying my DJI drone from the boat mainly in fear of losing it to the salt water.
It is kinda tricky taking off from a moving boat. I was in my daughter’s 19’ sailboat (which was under way at a measly 1 kt.), I lifted off from the seating area in the stern, and hovered for just a second, but the boat moved forward and a rear guy wire struck the stationary mini 3 pro and knocked it into the drink! Lesson learned (the hard way) Fortunately, I have the DJI Care express coverage!
To add, you cant be the one driving the car or boat. You would need someone else to drive while you operate the drone. You cannot do both at the same time.
What about flying from a moving bicycle or a moving skateboard? Do they qualify as "moving vehicles"? Very likely, yes. Would I need to ride the crossbar while somebody else operates the bike?
yes, you can as it is daylight. But, why would you? A video or photo of the eclipse will not work without a special filter and a telephoto lens and 400 feet high is not going to make any difference on the shot.
I am also considering having my DJI drone focused on the solar eclipse and I have different lenses that came as extras with my drone I recently purchased, I am just not sure exactly which lence to utilize.