Have these people got no imagination? "Well, a bathroom has water in it...like the sea...". Cold blue paint, bits of fishing net, depressed looking dried starfish, crappy model of a lighthouse, pictures of boats like the ones that you see in their thousands in car boot sales and pub walls. Go with the herd, eh?
If you have a seaside themed bathroom you have no taste. I bet Darth Vader has a nautical bathroom. He bobs around in his bath full of Matey bubbles using his loofa like a lightsabre imagining that he's floating somewhere off Weymouth.
Does your collection of shells remind you of your holidays? Shells are secreted by tasty, slimy, bogey looking animals and when they die you pick up what's left and stick them on you windowsills. Classy. If I were to do that with sheep skulls I'd get called weird.
Please redecorate your boring bathrooms immediately.