A DC solid state relay would be best in this application, IMO. Even one of those cheapies from China will probably do. Or then do like Doctorandus_P said and roll your own with a MOSFET.
Otherwise, get a conventional relay that is rated for DC. The AC current rating simply doesn't apply.
Contacts tend to be subject to much higher arcing on a DC circuit, if I am not mistaken.
That is correct. AC naturally extinguishes the arc (voltage crosses zero periodically), so a DC arc is harder to extinguish. See:
That's why it's important to get a relay with a DC current rating if you're using it for DC.
Also, am I missing something, or are you trying to power a 24V pump from 12V?