Paulduino:
Humm. Do you think that 3144 Hall effect sensors could drive a 5V pulse signal up 20 meters of shielded wire to the arduinos?
Yeh, probably....
I'm sort of hesitating here because I'm more used to making sure stuff is pretty bomb-proof and the cost of parts is swamped by the engineering and/or overall system costs. If there's a need to run any signal down more than a foot or two of cable then you usually think of some sort of line driver and protection stuff for both ends.
In this case tho, it's not going to be in an electrically noisy environment, the 3144 has it's own regulator and 25mA output drive so I guess it'd be ok.
I would put the pull-up resistor at the 'arduino end' instead of attaching it to the device, then you are sort of 'current' sensing rather than 'voltage' sensing down the wire. On the 3144, I'd add a cap on the power supply.
You can then use a 12v supply for the 3144's via 64 x 100ohm resistors, giving some resilience in case any sensor goes short for some reason.
Using opto-isolators as inputs to the Arduinos would give you electrical isolation between all that stuff in the water and your delicate 'brains'.
Yours,
TonyWilk