any seawater or water flow switch will work. they make them for spas, and for marine applications (most common) so you know if the sea water quits coming in, to shut the motor down to not overheat it.
a complete kit with alarm can be had for under 100 EASY. and as low as 50 that comes with a buzzer and an alarm light to really let you know when the shiet hits the fan.
https://www.aqualarm.net/comerus/sto...?idCategory=67
some examples, and those prices are far from the cheapest ive found for the exact same thing.
i prefer the straight line over the elbow. not that it REALLY matters much, but water technically flows faster straight than around a corner. possible flow restriction?
ive even seen clear ones.
6560-857, Sundance Spas Flow Switch, 1 Pump Systems
kinda cool.
i know this is old, but hopefully helps someone.
