As far as I know, the S5 Active is only available as an AT&T phone, which means it will have a locked bootloader and any somewhat newer firmwares will be unrootable. Pre-lollipop builds (Anything below OF2) are rootable but still have a locked bootloader.
Just a word from the wise, don't get an S5 Active, and DEFINITELY don't get an AT&T phone. a GS5 (regular) with a thin case (Spigen slim armor or neo-hybrid/ otterbox commuter) will be WAY more durable anyway, plus it's still waterproof/dustproof.
Get yourself a G900T (T-Mobile variant), or a G900AZ (Cricket wireless variant --Cricket is a subsidiary of at&t--), or even a G900F. Those all have unlocked bootloaders and will allow you to root, install custom recovery, custom roms, whatever you want forever. I personally have a G900T using it on AT&T with no issues, as the G900T supports all the same frequencies (and an extra one!) as the G900A (AT&T model). The G900F doesn't support every single band AT&T uses, so I would pick one of the other two if you want 100% compatibility.
MAKE SURE if you buy a G900T or G900AZ that it is UNLOCKED
or you won't be able to use it. And after you get it, you'll have to call AT&T support and give them the new phone's IMEI number in order for LTE to work.
Hope what I said makes sense..
FYI The best place to buy a phone is always (XDA's own..) Swappa!