Hi,
I am looking for a way of having a central Arduino that takes water sensor readings from multiple places around the garden.
So, Bluetooth is my first thought with the Arduino/Master Bluetooth polling about 8 remote devices, taking a reading and then doing something else based on the reading.
No wires between the remote sensors and the master Arduino but the remote sensors could have battery/ solar power.
NRF24 is made for this sort of thing and makes more sense than Bluetooth. They have about the same price and performance. Your central Arduino may also have Bluetooth to communicate with the outside world.
A likely better bet is to use little ESP8266 WiFi modules in a network with a Node-MCU or maybe a PC. No Arduinos - anywhere.
This Simple nRF24L01+ Tutorial may be of interest. It includes an example for a master and 2 slaves that can easily be extended to a larger number of slaves.