Please give me some tips to paint water/sea/waves better

You have lovely colour choices and tonal variation happening! One tip I could give is to do some of the water in layers, using a wet on wet application you can create edges that look a bit like the layering of waves on the beachfront. Also utilising my transparency (less pigment) on the shoreline and more toward the 'deep' water will give the illusion of the deepening waters :) start with your least pigmented later, let it dry then work your way out :)