Not sure about the material, but the Outdoor Uke is worth looking into. Had mixed reviews when they were first released, but I hear they are much better now and fit your criteria.
I have the Outdoor Ukulele tenor and really enjoy using it. I keep it in my car and take it everywhere. It is a polycarbonate, not carbon fiber. The tenor is $145 and the soprano is $95. http://www.outdoorukulele.com/collections/ukuleles
+1 for Martin. Their 0XK is a lovely little instrument. It's the perfect uke for travel and conditions that I wouldn't think of taking my "nicer" instruments to. I'd never get rid of mine.
I've not played any other carbon ukulele as none of the other carbon instruments makers -- to my knowledge -- offer ukes or do them in any volume.
Not sure if you're leaning tenor or concert, but I owned a Clara and it is a lovely instrument. They not offer an eKoa tenor, but eKoa is not quite a sturdy as the carbon fiber BTU if that's any concern. The latest BTU tenors are coming out really nice as they continued to refine them. Check out the sound samples on HMS if you don't have access to a local dealer.
They seem to be a bit of a secret round these parts but I would buy a Bonanza ukulele before an outdoor uke or a Martin OXK. Still not carbon fiber but also made from HPL laminate with much more selection than fake koa or fake bamboo.
I think Carbon fiber leans on the expensive side of the beach-proof ukulele market.
If you're simply looking for a cheap beater to take to the beach, you can get a plastic ukulele, such as the Keealoha plastic uke, Kala Waterman or the Bugsgear Aqulele.
Ironically enough, as great as Carbon fiber ukes, such as the Blackbird is... at that price, I would:
1. Not be willing to take it along to a beach as a beater.. that's thing's expensive!
2. Probably get a high-end koa uke
I'd really like it to sound good (I´m coming from a guitar and violin background), so carbon is hard to beat for sound/durability ratio, even if its expensive.