Reducing Noise from the Sensor Signal

I am using 18 submersible water level sensor in a floating dock. Maximum distance of the sensor from controller is 100 meter. I have made a Data acquisition card to pick the 4-20 mA signal from sensor. But as it is a noisy area i think some noise is adding with the main signal and making some problem to get the accurate result. What can I do for this?
> I have made a Data acquisition card
A made-in-the-shop card? Or a commercial card? What brand and model?Stand-alone or PC-based?

> But as it is a noisy area
Audio noise doesn't count. What's a likely source for electrical noise? Electric arc welding? VFD's?

>4-20 mA signal from sensor
Are these commercial submersible transmitters? 2 wire, loop powered 4-20mA?
2 wire xmtrs are isolated from ground.

What are you doing with the cable shields?

Is the only one DC power supply for the transmitters? Is the DC power supply floating or grounded?