Because of the 700 foot distance to the furthest waterer and the fact that two other waterers are sharing the first half of that distance, you will need either 8 gauge wire all the way, or 6 gauge for the first half, and 10 gauge for the second half of that run for the 1.7 amp (200 watt) waterers as-is.

Twelve or fourteen gauge pigtails between the 6 or 8 gauge line and a GFCI unit (to fit under the terminal screws), and even a 14 gauge pigtail between the line entering the panel and its breaker will not introduce or re-introduce voltage drop problems.

If you can use float switches only in the field and control the water flow back at the barn using contactors and valves, get contactors whose control runs on 120 volts, not 12 or 24. If the contactor control (its coil) needs 30 watts @ 120 volts, (3 contactors use 90 watts) you can run 14 gauge wire throughout. (You need 4 conductors, three control lines and one common line, arriving back at the barn.) Now if you can get valves that operate on just 30 watts then you won't need contactors.

Using low voltage will result in much worse voltage drop problems. For a given number of amperes and a given piece of wire, the same number of volts, not the same percentage of volts, is lost. Also consider that, for the same number of needed watts, lower voltage means higher amperage.