High Definition Photography A Few Steps Further Landscape /underwater Photography?

Landscape /underwater photography? - high definition photography a few steps further

What's the Dealio with cameras.
If I were a landscape photograph for each underwater photography, or what should I search?
If more megapixels = better resolution is more than the definition of equality in the picture?
"Can anyone recommend camera under $ 400, or special lenses or filters? A polarizing filter is useful in the landscape? expensive?
Is that on too many questions: |?