1) You can use an HC-04 ultrasonic sensor to measure the water level. This sensor cannot be interfaced directly with the Bolt, but you can connect this sensor to an Arduino and then interface the Arduino to the Bolt.
2) These sensors can be kept about 2 to 3 meters away from the controller.
3) If you are planning to use multiple HC-04 sensors, yes you can hook up multiple of such sensors to the Arduino and then interface the Arduino with the Bolt.
If you are planning to interface multiple sensors directly to the Bolt, you should know that the Bolt simultanesouly supports upto 5 digital Sensors (Once that only output 1 of 2 levels and do not require communication) and 1 Analog sensor (Sensors which output voltage in the range of 0 to 1 v).