To be honest, it would be quite a long list so I would just enjoy the scenery and sea air.
Try a ND or gradation filter instead of using f/20 and decide what's important: clouds or sea. At the moment I feel you leave the decision to the one looking at your picture and all I can say: I see not enough of both and too much of both. Make up your mind.
Treating the existing shot in RAW............ firstly..... is it level?.................in Adobe RAW.......
............ using the "adjustment brush" I would bring up the light in he creamy sea moving around the rocks and increase the "clarity" to give the sea a little more sparkle..................... Generally more punch is needed and white in the snow covered mountains. I would use multiple adjustment brush "pins" so each area/subject can have it's own settings.................the general rule is all shots should have at least one part fully white no matter how small and one part black,
..........add sugar to taste.............all is not lost there!
I agree with JoJu with regards to the composition, too much sky. Perhaps also use a lens with better sun stars eg canon 16-35 (f4 or F2.8, both are great). Could it be shot a little earlier to get some sun on the mountains?
A polarizer doesn't do the same as an ND. With some ND you get 10 f-stops - more precise, you loose 10 f-stops of brightness and can shoot wide open in daylight or medium wide open with longer times. There are different brands available, the more expensive ones not necessarily being better than some cheap ones.
I meant, point the camera higher towards the sky or lower to get more foreground. It's better in this case to use higher ground if possible. Don't take unnecesary risks.