I have no idea what to do with this?

Okay, well, my sister being stupid yet thoughtful of me went to the beach with my mom the other day while I was at a friend's sleep-over and my sister brought back three sand crabs with her, they aren't very big but they are still sand crabs. My birthday is in a few days and she wanted to give them to me as a b-day gift.. I'm scared, I don't want them to die. She's a lot younger than me (4) and she had no idea what she was doing was wrong. She brought me back three water bottles full of salt water from the beach and a sandwich bag amount of beach sand. I am currently keeping them in a 2 gallon fish bowl I used to use as a (well, you guessed it) fish bowl! Haha, lol... Durhhh I guess... Anyway, I put in the sandwich bag of sand and two water bottles and the sand is just moist and the crabs are like digging aroung and everything. I have one water bottle of salt water left, and I have no beach sand left. We live an hour from the beach in a part of San Diego (inland area) and we rarely go to the beach, so I can't bring them back to their home): I feel really bad for them because I don't want them to die. I've had them since Friday. Do they eat sea weed or what? Please don't give me some answer that I'm not looking for, like I'm making the wrong mistake or something for keeping them because there is nothing I can do. If I run out of water, what do I do?! PLease help me...!