HI!
I`m building an ROV sub, and as i am an Arduino noob and need help writing some code.
the 6 rov motors are currently controlled with on and off switches in an arcade joystick, and relays.
I want to reduce the number of cables, an im hoping to use the rx/tx on the arduino. I will have one arduino on the surface and one in the water.
can someone please help me getting started with a simple code that I can build upon?
" push a button connected to the surface ardunio , and make the other arduni write a pin high"
im sure it is super simple. (not for me) 
thanks from Norway. 
Aren't you going to post a photo of the sub ?
Below simple serial code for sending data from one arduino to another. An ethernet connection might be of interest if video from the ROV is desired.
//zoomkat 3-5-12 simple delimited ',' string tx/rx
//from serial port input (via serial monitor)
//and print result out serial port
//Connect the sending arduino rx pin to the receiving arduino rx pin.
//Connect the arduino grounds together.
//What is sent to the tx arduino is received on the rx arduino.
//Open serial monitor on both arduinos to test
String readString;
void setup() {
Serial.begin(9600);
Serial.println("serial delimit test 1.0"); // so I can keep track of what is loaded
}
void loop() {
//expect a string like wer,qwe rty,123 456,hyre kjhg,
//or like hello world,who are you?,bye!,
if (Serial.available()) {
char c = Serial.read(); //gets one byte from serial buffer
if (c == ',') {
if (readString.length() >1) {
Serial.print(readString); //prints string to serial port out
Serial.println(','); //prints delimiting ","
//do stuff with the captured readString
readString=""; //clears variable for new input
}
}
else {
readString += c; //makes the string readString
}
}
}
You want to use RS485 over the link because you're going to have a long tether and the 0V to 5V, non differential signaling of TTL serial is much more susceptible to interference. To implement RS485 it just requires a couple of MAX485 (or equivalent) chips which are quite inexpensive, and with twisted pair wires you're good for a few thousand feet of tether.
http://johanneskinzig.de/hardware/arduino-communicates-via.html
For the software side I'd suggest the EasyTransfer library. What it does is just keep a struct synchronized between two Arduinos -- kinda like both sides are reading the same variables -- and handles bad data (performs checksumming) for you.
What range does TTL work? Is that just 2 wire Rx & Tx?
[quote author=Andy R link=topic=240470.msg1727619#msg1727619 date=1400362624]
What range does TTL work? Is that just 2 wire Rx & Tx?[/quote]
You usually also have a ground wire with a tx/rx setup.