Billabong, Volcom, Quiksilver - closing all US stores, 60% off site wide (including sale) with code SITEWIDE - free shipping
I haven't worn any of these brands in the last 20 years and I personally wouldn't start now but there are some good deals on flip flops, backpacks, swim shorts, accessories, etc.
I found some good swim trunks that came out to ~$15 but I did have to wade through (pun intended) a bunch of 10" inseam shorts to find them. For some reason these brands use "outseam" and not inseam but I measured other trunks that I had and it looks like their 16" outseam shorts are 7" inseam.
For free shipping with no minimum you need to create a "loyalty" account but that just means signing up with an email.
They're having the same deal for Roxy if you want to get your girl some swim stuff.