I need help with some way to make rocky waters along a coastline to represent shallow deadly seas which is too shallow for ships to navigate.

I use Photoshop - but im no expert and have no idea how i could make it look good, the view is 2D top down (birds eye view). I have attatched a snippet of the corner of my map to show the current coast line,
I want to fill the blue ocean area with a rocky appearance without vegetation.

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Any suggestions on how I could make something look good that matches the style in Photoshop?