As many others post, the idea proposed is dangerous and a code violation and therefore not to be considered.
As to the THEORETICAL argument as to whether or not it would work, there is no straight yes/no answer to this.
Presuming a standard 3 phase, 4 wire supply with grounded neutral at 120/208 volts, but with only 2 phases at the light pole to supply a 208 volt lamp.
If there is a ground wire at the pole, and this is continous back to the ground bar at the service entrance, then a 120 volt load should work if wired between either hot wire and the ground.
This Would be DANGEROUS and a code violation and should not be considered, though it is regretably common in some parts of the world.
If there is no ground wire at the light pole, then that is almost certainly a dangerous code violation already, without adding anything.
Connecting a 120 volt load between a local ground rod and one phase, is not only DANGEROUS but is unlikely to work since the resistance of the ground connection is too great for any significant current.
If the current is very small, then it might work, but is STILL DANGEROUS and a code violation.
To return to the original question, the best answer would to run a new circuit to code, suitable for the proposed pump.
If this cant be afforded, then it might be possible to fit a transformer in the light pole to obtain 120 volts from 208, with a ground rod to ground the neutral of what is now an SDS, albeit a very small one.
This presumes that the existing wiring is to code and adeqaute for the present existing and proposed load. The low voltage suggests that this may not be the case.
If the pump is only required occaisionly, not all the time, then a gasoline powered portable pump, or a perhaps a 12 volt pump from a vehicle battery might be worth considering. If for an ornamental fountain, then a PV pump might be worth it.
