-does the analog read function only read up to 5V ? would more damage it ?
Yes pin voltage in excess of Vcc voltage will cause damage, so 5vdc limit on 5 volt boards and 3.3 volt limit on boards running a 3.3vdc.
-i need the pump to cut out @ 10.5V
Your choice, but you might want to check battery manufacture data for safe discharge voltage to be sure.
-i was thinking a voltage divider could be used to cut the battery voltage in half
Yes, a typical solution. Wire a 10k ohm and a 5k ohm resistors in series, 5k end to arduino ground and battery ground, 10k end to battery positive. Resistor junction wires to analog input pin. Scale the analogRead counts to a useful scale say in millivolts.
-im thinking a delay would be needed after it cuts out the pump, due to the voltage rising fast after the load disappears
Better method is called adding hysteresis in the switch states, so you need two decision values in your code so as to not chatter the relay on and off at a single voltage set-point. You should also wire the relay such that it is normally power on when battery voltage is OK and turned off with battery voltage is low, so that you are not consuming battery current for the relay coil in the low condition. Using a relay with normally closed and normally open contact options make this easy to do.
-where in the code would i put the delay command ?
I don't see your code? Actually using proper two values with proper hysteresis you shouldn't need a delay. Relay turns off power at your 10.5 value, but not turn it back on until 11.5, for example. You will have to play around with the values and proper separation values, but that's not hard to do.
Lefty