Since I had enough fabric, I decided to make a matching top. I went with a long sleeved, envelope neck T-shirt. I wanted the extra coverage for sun protection. I used McCall's 6103 for the top, but you could easily make a pattern yourself using this Made tutorial.
I guess I wasn't the only one thinking about swim diapers, because there's a great tutorial on Sew, Mama, Sew! as well. Their swim diaper is either a pull-on, or a side snapping diaper. I don't like the idea of a pull-on, and I don't have a KAM snap press/pliers (yet!), so I decided to go ahead with my original idea. You should go over there and take a look at the pictures. They can be really helpful if you've never used fold-over-elastic.
I used the Chloe Toes free diaper pattern to make the bottoms. I made the preemie size, because it's just going on a bare butt, no prefold or other soaker layer. The fabrics I used were a swimwear fabric outer, an inner layer of *polyurethane laminate fabric (PUL), and a layer of mesh. I got the mesh at Joann's, in the utility fabric section. You don't want to get the netting that they have in the apparel fabric section. It's not sturdy enough. You will also need hook and loop tape and fold-over elastic (FOE). You can get all of your supplies at Joann's, that's where I got mine, with coupons of course.
Cut a patter piece from each of your 3 fabrics. I wanted some butt ruffles, who doesn't, so I cut two 1" wide strips from the selvage edge of the fabric. The selvage edge was white, so I thought it would look cuter than cutting the strips from the printed part.
Sew a long running stitch down the ruffle strips (the length and number of strips you will need will vary depending on which size diaper you sew). Lay your PUL, shiny side up, and put your outer fabric (in my case, swimwear fabric) right side up, on top of the PUL. Arrange your ruffle strips on the back end of the pattern and pin. Sew a straight stitch down the middle of the ruffles.
Place a strip of loop across the front of the pattern, and sew down both long edges using a straight stitch.
Both the ruffles and the loop tape are sewn to the outer fabric and the PUL. Don't sew those two things just to the outer fabric, especially if your outer fabric is a knit, because it won't be as sturdy and it could warp the shape of your outer fabric.
Pin the mesh pattern piece to the other fabrics, so that the mesh is next to the PUL. Don't pin the back wings of the pieces together.
Cut two 1" pieces of loop tape and sew them to the back wings of the fabric. Only sew them to the mesh and the PUL, leaving your outer fabric free. These will be your laundry tabs, and will keep the hook/loop tape from snagging anything when washed.
Pin all three layers together, marking the elastic placement points noted on the pattern. Sew your fold-over-elastic all the way around your fabrics, making sure to stretch it between those elastic placement points. Check out the tutorial I linked to if you have never used FOE. There are also some great video tutorials on youtube showing how to use it.
Cut four 1.5" pieces of hook/loop. Two pieces from loop side and two pieces from the hook side. Round one end on each of the pieces, and do a nicer job than I did.
You will sew these on to the ends of the back wings. The loop part will be on top of the outer fabric and the hook part will be against the mesh, so that the wing is sandwiched between the two pieces. Sew all around the tabs using a straight stitch and then zigzag around to keep the hook/loop tape from fraying.
Here's an action shot of the laundry tabs. Pretty neat, huh?
Here's Penelope modeling her new swimwear. From the waist up, she looks like a Duggar.
*You can get PUL at Joann's, in the utility fabric section. I've heard that their PUL is of crappy quality if you use it for covers which require constant washing. Since this is just a swim diaper, I think their PUL would work.