I have redwood decking that is screwed down around a redwood hottub. I need to remove the decking. I tried to unscrew the screws on reverse mode with a Black and Decker 3.5 amp power hand drill. The screws don't budge. Do I need more power? What kind of hand drill would do the job?

If that won't do it, a More powerful Drill will, I have Dewalt and Makita, others will work as well.
Put Drill on Lowest speed, and 1 of 3 things will happen, screw will break, screw will be removed and screw can strip.
They make stripped screw removers.
What kind of screw head?

Stratmando, it's a philip's.

Be sure to use a new tip, have no debris in phillips head to ensure good contact, apply good pressure to avoid stripping.
Will see if I can find the extractors.
Here's the extractor.
It may require the Blue phillips(for Deck Screws)

When that happens I just use a screw driver and a ratchet plier.

http://www.plug-sockets.com/productsimages/ratchetmechanicalcrimpingpliers_114299.jpg

I attach the plier to the screwdriver handle and try to unscrew using the plier as leverage.

Here's the tips I was talking about:
eBay (http://cgi.ebay.com/DECK-MATE-BLUE-1%22-%26-2%22--PHILIPS-SQUARE-DRIV-BITS--_W0QQitemZ290319426711QQcmdZViewItem)
I think I would use my 18 Volt Makita Impact w/lithium ion batteries.