Hi. We'll be creating a ROV for our robotics class. We'll be using a 12v 2200 kv outrunner brushless motor for the thrusters. Will it be necessary to use an ESC? If yes what ESC should we use? 
Thank you
Yes use an ESC, use one that's well able to handle the load current. You haven't given much information
to go on...
This are the specs of the brushless motor we'll be using.
KV: 2200rpm/V
Max Efficiency: 80%
Max Efficiency Current: 4-10A (>75%)
Current Capacity: 12A/60s
will a 20A ESC be enough for this? thank you
Joshinite:
will a 20A ESC be enough for this? thank you
A 20A ESC would be appropriate.
As an aside, the Kv rating of your motors is pretty high for use as thrusters.
Chagrin:
As an aside, the Kv rating of your motors is pretty high for use as thrusters.
How about a 1000kv motor? Do you think it would be better?
Joshinite:
How about a 1000kv motor? Do you think it would be better?
Yes. As a comparison, the OpenROV uses 700Kv motors.