Water Meter Monitoring
Voltage Drops

Pulse counter has a high impedance input (probably).  So, there will be practically no current through the signal wire.  Therefore there will be no voltage drop across the signal wire.

I'm assuming that you're planning to make a 4-terminal system (power, GND, irrigation signal, potable signal).  At the house, your signal will be measured with respect to, GND.  Voltage drop across the GND wire will be added to the signal (as a DC offset).  Power supply current is running through GND wire.  We can't assume that this current will be negligible.  You should measure it.  Resistance of 18AWG wire is 6.4 Ohm per 1000ft (from tables).  If the circuitry at the end of 200ft wire consumes 100mA (for example), it will cause a 0.128V drop in the GND line.

Your pulse signal is a pseudo-digital voltage signal.  To an extent, it's tolerant to voltage drops across the signal and ground wires.  As long as the pulse signal is greater than a threshold, the counter will count it.

Interference

For a 200ft line interference can become a serious problem.  Usually, the strongest source of interference is 60Hz from AC power lines.  If you hang your cable it along the power lines, it would be the worst case.  Cable with properly grounding shield helps.  Some signaling methods are less susceptible: 4-20mA current loop, differential signaling. For the time being, I will not go into more details here.

Lightning Protection

Gas Discharge Tube (GDT) or Spark Gaps.

Optoisolator makes sense.  Keep in mind that power supply and GND lines are also susceptible to lightning.  To isolate the the exposed part of your circuit completely, use an isolated power supply.