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I saw fiddler crabs along the beach when I visited Galveston. There were hundreds of them in the back part of the beach where there were plants and places to hide a bit. They don't spend all their time in water, but when they dig holes in the sand, they are in a wet area. The tide may come in over them.
If you put your crabs in salt water, they won't last too long, a few weeks maybe. If you put them in a wide shallow bowl and put some rocks in where they can come out of the water, they'll live much longer (use half salt water, half RO or distilled water. You'll need to change the water every few days.
If you want to make them really happy, take a plastic box and fill it with a few inche of sand, then sink a bowl of water (1/2 salt water, like I described above). Put some shells around, pour enough of the water so that the sand is damp but water doesn't stand in the bottom.
Feed them marine flakes, lettuce, pieces of fruit, and air popped popcorn. We've kept some for about 16 months like that. They only thing is, they're so happy they come out only at night. You can see their tracks in the morning.
Clownfish, I use distilled water for my crabs (instead of RO water) because its easy to find and the gallon size is easy to pour. You can even buy distilled water in gallon jugs in the grocery store, so it's quite convenient. Usually costs just over a dollar per gallon. You can use distilled water to make salt water but most people use RO water since it's available in gallon jugs from many places (I think we found it at Walmart).