What are you using to control the pump? You would only require a very small esc.
A power resistor would limit current, it is more wasteful than chopping the current, which is what an ESC would do, plus putting heat into a cylinder isn't my favourite solution. However, as you correctly point out, you are only likely to run the pump for short periods. Finding the right resistor rating will require a bit of trial and error. You might want to start with a 10 ohm power resistor of 4 watts rating or higher.