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 :)

In addition to what the other commenter said, you can go in with some more opaque white afterwards for sea foam/reflection detail

I know that although most things lose colour strength as they get further away, the ocean is usually different.

So you could do darker water further from shore. Lighter closer to shore.