If you want to use serial communications:
Keep you baud-rate low, like 9600. That is a good trade-off between speed and transmission quality.
If you are using the serial-line of an arduino, that is a 3.3V or 5V line. Over 50m you may have an issue.
If you use a RS232-chip (like MAX2323), the chip usually bumps the voltage up to +/- 12V. You can easily communicate over 50m at low baud rates.
Have a look at the data sheet:
http://www.ti.com/lit/ds/symlink/max3232.pdfYou would have to have something like an Arduino in both your top-side controller and your ROV.