@mf2105 see page 14 of this manual for a cool peel and stick level sensor.
http://www.rvgauge.com/downloads/709_Shop_Manual.pdf
I think it works based on capacitance or capacitive reactance.
You could also attach a bunch of these cheap float switches to a stick and just submerge the whole thing in your barrel:
http://www.ebay.com/itm/Hot-PP-Side-Mount-Horizontal-Level-Sensor-Liquid-Float-Switch-for-Tank-Pool-EA9-/271295779400?pt=LH_DefaultDomain_0&var=&hash=item3f2a7d1248
Since they would float in sequence, you can create a type of resistor ladder with them and sense your level with just one Analog input. Picture a 20k pull down resistor, and a 100k pull up resistor on an analog input of your choice. With each float switch, you could connect another 100k ohm resistor to 3.3V. The effect would put more and more 100k ohm resistors in parallel as your water level rose. Effectively raising the voltage at the same time. Easy to calculate the voltages.
1 sensor = 3.3V x 20k / (((100k + 100k) / 2) + 20k) = 0.94V
2 sensors = 3.3V x 20k / (((100k + 100k + 100k) / 3) + 20k) = 1.24V
3 sensors = 3.3V x 20k / (((100k + 100k + 100k + 100k) / 4) + 20k) = 1.47V
4 sensors = 3.3V x 20k / (((100k + 100k + 100k + 100k + 100k) / 5) + 20k) = 1.65V
N sensors = 3.3V x 20k / ((100k / (N + 1)) + 20k)
Sensor Voltage
1 0.94
2 1.24
3 1.47
4 1.65
5 1.80
6 1.93
7 2.03
8 2.12
9 2.20
10 2.27
11 2.33
12 2.38