Lobster Designs

Shell against a background that is more or less the sea. Turquoise blues and light sand colours, as in this wallpaper. Possibly reverse colours so that shell is vivid sea blue and sea is sandy coloured, and perhaps blend irregularly back into the shell colours for a border. Traditional quilting pattern used for the background, may only be partial e.g. diagonal lines with occasional short rows of triangles, or Storm at Sea. Try to get the shell to start fading into the sea using this pattern. This is easier with large pieces for the shell, but don't want the background to end up with fiendishly small pieces. Play with transparency if the shell fades into the sea. Originally thinking a nautilus shell, perhaps something like this so that there is shell detail and still plenty of space for the blending into the sea.

For the background, something with more movement that the traditional square blocks. Storm at Sea can get a decent curvy diagonal going, e.g. here, though it's terribly regular - stretch it in areas? This is simple but with nice lines. Going back to SaS, I like this - colours don't work for me but the way they're blended does, if it could be converted to a better colour scheme. This is another way of showing diagonals. This has its moments too. Interesting variant in reds.

Ocean waves has possibilities too (another version), as does Lady of the Lake, although again that's quite a big-scale pattern.

Kaleidoscope might do as a smaller alternative to SaS. Examples:
one, two, three.
Consider the placement of the shell, how it is interacting with its background.

On the other hand, something simple using diagonals, as very vaguely sketched here, might work better.

This quilt artist does this sort of thing.