You'll need at least
2.5 psi to top a raft, but even that probably won't be enough. We use this
blower with the 2.75psi limiting valve at the company I work at (I know, it's not 12V, it's 120V) and I still feel like the boat is soft, and needs a hand pump for topping.
My advise for recreational use, and for 12v use is to go to Walmart and pick out a cheap one that will blow the bulk of the air into the boat, and use a hand pump to top it off. You're going to need a hand pump anyway, to take down the river. I've gone through several like these
from Walmart- never burned one out, always lost 'em somehow. Now I'm to the point that I just use my Bravo II
footpump for the whole 18' raft. Takes a few minutes, but with the foot pump at least I can drink beer while I pump.