I see the lightening strike and while it's cool.. it doesn't seem to fit with the rest of the painting. It's all I can see and I'm left wondering where the rest of the storm is. Make some seriously heavy clouds.. darks.. black and a green ugly angry sky to go with that. Make it feel like it's pressing you down into the wet sand and you can't breathe for seeing those clouds.
I thought that you had captured the look and feel of the ocean lightening strike beautifully, on a captivating ocean landscape in soft twilight lighting. The softly difused horizon speaks of the closing of the day as the light fades and such lightning storms so often occur. I do agree with the earlier comment that the "flash " could be longer making a slightly more dramatic impact but as this is a work in progress I just wanted to say that you have made a wonderful start and it will be interesting to see the final outcome.
I like the lightning. Yes a top and bottom would be good. I don't have a problem with the slash marks. They will probalby look different anyway if you darken the top. As for the bottom, I like it but maybe darken the bottom half or two thirds of the brown with something shiny to match the blue part. I assume the brown is matt and the top satin finish? Not sure how much detail is required, I like the abstract feel of this.
maybe a focal point? not overwhelming so you lose the wonderful color-meld you've created, or maybe some text up one side of a poem relevant to the scene. Good luck. It's great so far.