larsgregersen:
It is clearly the wrong MOSFET for the job. The resistance is way to high when you can only switch it on with 3.3 V (not matter how fast you can do it - speed is another problem).
I know this is splitting hairs, and I completely agree with your wrong MOSFET assessment, but the OP reported that he/she was able to control the pump by manually toggling voltage at the Gate, so that suggests that the MOSFET can, with even 3.3V at the Gate, supply proper current to the pump after everything settles. So, it seems to me that speed of switching is a primary issue, and the cause of this deficiency is the poor performance of this MOSFET at 0 to 3.3V levels at the Gate.
Same conclusion, different reason.