For a great long exposure picture the camera needs to remain absolutely static, and of course shooting it with a drone is different and more complicated compared to shooting from the ground with your camera on a tripod. When you fly your drone, it is under the influence of wind, interferences and other environmental conditions that you can’t manage...so one of the most important things is to check accuracy weather conditions of the location you want to capture. As you can see below, when everything is ok you will be able to take amazing pics and have a lot of fun!
This is the stunning natural thermal pool of Saturnia, Italy... conditions were almost perfect: no wind, no interferences, nobody in the water... and an amazing sunrise! The greatest moments to take a long expo picture are at sunrise or sunset because of the soft light...that along with a ND filter allows you to get a nice result.
This pic was taken with a ND 16 Dji filter which I highly recommend you, it allows you to open the shutter for seconds without overexposing the image too much. It definetely makes the difference for your footage too! Without a ND filter, you won’t be able to take a picture like this...
There are different filters, with more and less stops of light... I personally own ND4, ND8, ND16 and ND32 filters, to be prepared for every situation!
When flying your drone above the water, especially if you want to take a long exposure picture be sure to turn off VPS or downward sensors. Patterns like water doesn’t let your drone hovering stabilized if they are enabled.
Try shooting from different angles around your subject, you may be surprised of what your drone can capture...
Consider to shoot long exposure above the sea, its waves create some artistic appearance...
Shooting by night is also amazing because you can play with artificial lights.... moving cars become colorful lines and of course you don’t need any filter to get this shot. Try using colorful flashlights in a dark site and let your imagination run free!