If you are sure to offer the ocean salt water and keep it fresh, there's no need to sprinkle it on their food. They would do better with crushed cuttlebone or spirulina sprinkled on their food.
Sometimes you can find spirulina in a natural food store or in the natural food section of your grocery store, in bulk bins or containers where the bulk spices are sold. Mine has stopped carrying the spirulina, and I refuse to buy an expensive bottle of caplets meant for human consumption! In bulk, the price is almost nonexistent. You may also find kelp granules available there as well. Kelp can be served wet - sometimes I mix it with some dechlor water, or even some olive oil, as they seem to avoid it when it's dry. The spirulina is another matter - it dries up hard if you wet it, so I sprinkle it over foods.
You can pick up seaweed at the pet store in the fish food section. There are sheets of it you can crumble up in a blender and store in the fridge (as Marie does) to sprinkle in their food. This is sometimes labeled as algae - red, brown, and green. You can purchase it on line as well - Marie uses Three Little Fishies brand.
You can also order seaweed mixes online from places like Keep on Crabbin'. This is owned by a member of this forum (Spidtat) who makes all organic mixes, so you know your crab's food is safe. he also sells some of the items from Crabotanicals. I highly recommend them for their healthy mixes, especially for foods you can't find easily, or it would cost a lot for you to mix your own by buying all the ingredients.