Smelt (Moto 360 gen 2 46mm), carp (42mm) and bowfin (Moto 360 sport) are a series of round watches with flat-tire LCD displays. A plastic ring must be removed to expose USB pads, but this doesn't compromise water resistance.
Smelt and carp are round steel-cased watches with a fully glass front and one button. They have a 'flat tire' display, where the bottom section is cut off and houses a brightness sensor. The display is a backlit LCD, so using it with Always on display is not recommended as power draw gets quite high. The watches charge wirelessly. Smelt takes a 22mm strap, carp has two variants, the 'mens/default' version taking 20mm straps and the women's version taking 16mm straps. The default straps for smelt and carp are really nice, with a selection of steel bracelets and high-quality leather.
Moto 360 sport
The moto 360 sport (bowfin) is an interesting variant of carp that comes with the addition of GPS, a transflective display and a rubber unibody case with non-removable straps. The transflective display is a variant of an LCD that does not need a constant backlight, so is readable when the backlight is off and even in sunlight. The unibody rubber case is bulkier than the metal case of standard carp, and the straps are known to crack with use. Replacement rubber casings aren't common, but it may be possible to swap a bowfin into the case from a carp (though nobody has tried this swap yet).
USB is exposed via a set of 4 metal contacts hidden under a plastic ring on the back of the watch. Removing only this ring won't comporomise water resistance. The ring should be quite easy to remove with just a spudger or knife and no heat - if you encounter significant resistance, check you're not trying to slice off the back glass. The layouts on smelt and carp are different. It is recommended to print a dock to make use of these pins or to use some clips.
A 3D printable dock for Smelt including space for a generic 44mm Qi-Charger pad can be found here https://www.thingiverse.com/thing:6251316
The disassembly process of these watches is really wild but thankfully there is no glue involved. It is recommended to take a look at the iFixit guide and some videos to gain a good understanding. Beware that the display ribbon cable is fragile when opening the two halves of the watch. Batteries are available as FW3L for smelt and FW3S for all versions of carp and are fairly common. You will likely need to replace the battery when buying these watches as they are getting quite old.
Miscellaneous hardware docs
Getting to fastboot
Keep holding the power button during the boot process and release it quickly after the third vibration.