Yes, it's possible to unlock (meaning the carrier unlock so you can use it with other cellular providers). I have a GS4A Dive Blue AT&T model and I used an eBay reseller to get an unlock code for $7, delivered in under 24 hours, punched it in no problem and I'm using the phone with T-Mobile (and using the AWS bands as well, there are instructions on how to "unlock" those bands as well).
There is a thread specifically about GS4A cases here:
Right now work is in progress on creating a ROM or ROMs for the International version of the GS4A aka the SGH-I9295, there is more info in the ROM subforums here for the GS4A, I'm not up to speed on all that because I have the AT&T US model which still has a locked bootloader which is basically preventing any useful development (and a bounty is under way to get that bootloader cracked open). So, if you have the International model, you might get lucky with the CM build that's in testing/development; if you have the AT&T US model, you're stuck like the rest of us.
The GS4A is not waterproof
nor is it actually marketed as such but people see and read and hear what they want to hear so that's that. The GS4A is IP67 compliant which is just some standards code which means it is water-resistant
up to 1m (that's about 3 feet for us Imperial bastards that can't stand Metric) for 30 mins. That means you can put the GS4A under the water (up to ~3 feet in depth) for ~30 minutes to shoot photos or do some video recording, but ~30 minutes is the time limit and ~3 feet is the depth limit. Any more usage on either parameter and it could end up hosing the phone (pun intended).
The IP67 also covers dust and dirt and whatever protection as well so it's basically NOT a rugged phone but will tolerate more nasty environments for brief periods of time than the standard GS4 will.
Hope that answers some questions. As of right now I'm not aware of any cases that are truly water-proof
or will protect the GS4A absolutely and 100% certainly, but I'm sure if you look hard enough you'll find some case that can do it, designed for generic smartphones overall.
And yes, there is a working somewhat permanent root for the GS4A as well, geohot's active root, and there's a thread about the usage somewhere in this GS4A subforum - and yes mine is rooted as well. The root survives resets hence me calling it "somewhat permanent" and the only way to unroot presently is to restore a factory ROM. The problem is that the only factory ROM (a one-click image) that's available pushes the GS4A back to build MF1 which was a pre-release ROM and if you use that for any reason, you'll be stuck on it possibly forever because it does not allow updates to newer builds from AT&T - and yes I'm referring specifically again to the SGH-I537 model.
Not too sure about the International model on that aspect, the info is probably out there someplace but I don't even read stuff about the SGH-I9295 since mine isn't compatible with the mods and ROMs being developed for that one at all.