Thanks for all the great feedback so far, a lot of folks have pointed out some things that I either glossed over or simply didn't think of.
Regarding the stability of the raft, I'll definitely be running some tests both on land and in the shallows to see where the tipping points are and determine the necessity of an outrigger. I like the idea of having a swing arm that can be removed for transport.
To the questions of whether or not it will load up on the roof of a Jetta, I plan to try a test load tonight when I get home from work to test that aspect of the design. Part of the plan is to use a 2x4 to help lever the frame up on to the roof rack so that I'm not grinding the frame against the side of the car. I'll be sure to post the pics of the load test.
Steering and power won't be an issue, at least initially, as I plan to give it a few runs on smaller lakes that have little to no current to fight against. I figure that a paddle will be enough to get me out to the middle of the lake, but I'll probably need to add an anchor of some sort to keep me there.
The barrels are held to the frame by lashing paracord through some eye-screws mounted in the bottom of the frame to tie in the ends and criss-cross the bottom of the barrels for additional support.
In my research, I also found an article
written by some folks that put 3 people on a similarly designed raft along with enough supplies for 3 days of floating. They loaded their 6'x9' raft on the roof of a small hatchback and were able to get to the water without a problem. A lot of my design inspiration came from their example and story.