You, too, can make a waterbed for your Barbie. You will need a gallon-size Ziploc bag , a water source , and a piece of fabric.
Step 1: Fill the Ziploc with water from your chosen water source. Seal the bag and dry it off.
Step 2: Lay the Ziploc in the bedroom of Barbie’s dream home or in a shoe box or just on the floor wherever it is you enjoy playing with your Barbie. Make sure those dumb sharp Barbie shoes are not underneath your Ziploc. This is important: do not lay the Ziploc down on the zipper side or the other factory-sealed side. Use one of the biggest and flattest sides (you will have two from which to choose) to be on the carpet/plastic/floor. The other biggest and flattest side will be facing up.
Step 3: Lay the fabric (two or more pieces, if you’re fancy) on top of the Ziploc so it’s just one big and watery pillow bed.
Step 4: Put your Barbie on the bed (again, watch those sharp little shoes and also the equally inexplicably sharp little hands and earrings).
Now, wasn’t that fun? Of course, now Barbie is sleeping. And the waterbed for Barbie is, well, kind of boring.
Life can be cruel and it isn’t always fun. Sometimes you go to all the trouble to make a waterbed for a Barbie and it feels like it was for absolutely nothing. Adults will say things like “it’s not the destination, it’s the journey” or “there’s more joy in work than in the product” and “brush your teeth.”
Yeah, well, sometimes, you knock yourself out making something nice for a friend and you just get bupkis. Sometimes: it really is for nothing. No, do not pretend you learned any kind of important lesson from filling a Ziploc bag with water; do not pretend it was fun; do not pretend you are not just a little bit hurt that Barbie didn’t even say “thanks for the waterbed.” 
So, sure. You, too, can make a waterbed for your Barbie. I just don’t recommend actually doing it.
1. Don’t use a sandwich-sized bag. Barbie is freakishly tall, proportionally speaking. She will not fit on a sandwich bag, like, not even diagonally.
2. Ask an adult for help! Not a stranger, though. Do NOT ask a stranger for help with this.
3. Barbie (Trademark Mattel, who did not give permission for the use and/or mention of Barbie TM in this post) is a thankless piece of plastic. You may as well make a tiny gratitude journal for her, you know what I’m saying? Sometimes beautiful people are nice. But sometimes they’re entitled, ungrateful wretches who should get a day job or something or at least do some of your chores or homework or help out a little, I mean, my god, Barbie TM, how freaking lazy does a piece of plastic have to be? And ungrateful, did I mention ungrateful? How many Barbies get their own damn waterbed? Damn it, Barbie TM, at least say “thank you.” You don’t even have to mean it. It takes absolutely nothing, FYI. Nothing.