I'm knitting a bikini. Is it safe to swim in it, or are these things merely for sunbathing?

You would either need to use an elastic yarn, as stated in the first link or sew in a fabric lining, after you're done with your knitting for everything to properly hold it's shape in the water.

It looks like the pattern calls for an elastic yarn and is made for stretching, so it should be safe to swim in, but for added security I would after wearing it out to sunbath a few times would add in a fabric liner to prevent further unwanted stretching, not to mention the fabric liner will make it all the more comfortable to wear!

Different types of yarn react differently to water. Some shrink (good) and some stretch (bad). So test it out first in the bath or shower before you go out to a public beach. The last thing you want is to have it stretch and show bits that you don't want showing.

after testing the fabric (see above comment), if you do think you can swim it you, you are going to want to line it with a material that is water/swim-friendly, like a spandex. That way you don't see through it.

I think it just be for looks/sunbathing

this pattern and yarn is perfect for swimming in.