I read this post a while ago and I've been mulling it over and....I'm gonna play devil's advocate and point out a couple things, if I may. It's meant in no way to be personal nor to be taken as ripping on your idea overall.
Considering the size of the trailer, we have a couple problems.
First off, if we use 24"X34" for a drum and submerge it halfway, it gives us a max volume of about 5 1/2 gallons. If we figure that we can fit 2 barrels wide and three long on top of the deck for transport, that gives us 6 barrels, or about 33 gallons to displace. Displacing fresh water at around 8 lbs per gallon, they would float about 260 lbs. What does the trailer alone weigh ?
Secondly, the porch you are talking about cantilevered off the back - what happens to the boat's trim when 2 people are sitting back there ?
As to bow-mounted power ?
1) forward facing tractor prop - Volvo has recently introduced their IPS to the rec market and builders are having to build skegs in front of the duoprop to keep them from getting beaten up. Of course, this completely messes with the clean flow that the technology was designed around in the first place...
2) rear facing (pusher) - assuming you want to go with something like a pair of bow-mounted trollers steered with a tie bar, you would need to speed up the outside motor in a turn or the outer pontoon would dip. I don't know if you've ever been in a boat that moves that way (heeling to the outside on a turn), but it's not pleasant to most people. Steering in general would be tough, and trying a hole shot would cause the bow to dip rather than climb, which would sink barrels that are already half-submerged.
I agree with glen's observations that both retractable road wheels and a barge hull might be some solid ways to do what you want. If we are dealing with a 4X8 footprint, you can get the most bouyancy out of a box. Put a bit of rise on the bow and it shouldn't take too much to get that box on plane. If we figure a 4X8 box drawing 6" of water can float around 1100 lbs, then losing even 20% to a punt-type bow still leaves us enough bouyancy to float all of the foamies weighed to date, maybe even at the same time, lol.
Sticking with no more than a 24V system and using transom mounted trolling motors will keep that weight down as well.
Last point on weight...you can't beat the power to weight ratio of an outboard with any electric configuration so we need to either allow for more weight and make it a design strength rather than a liability, or we need to reconsider how to power this craft.
All that being said, you can't take the tow vehicle on the water with you so why take the trailer ?
You will still have to get out of the car to unhitch and disconnect the tow harness and chains. Launching a boat is a piece of cake once you've done it a few times.
Like I said at the top though, this isn't about ripping on your idea: have you seen these ?
They are available in 12/24/36V and up to 160lb thrust:
http://www.minnkotamotors.com/products/ ... water.aspx
I saw them at a trade show a couple years ago and thought they'd make a great electric drive for an electric launch I wanted to build and market - a tried and true product already configured and ready to use. I emailed my rep about why MinnKota 'doesn't recommend' using them as the primary drive system for small craft when they are used specifically for that purpose on larger and heavier boats.
The response that I got (actually from MinnKota themselves) is that they "don't recommend it because it voids warranty coverage"
I figured it would be pointless to ask WHY it "voids warranty coverage" ::hammerhead:
NO CAR NEEDED ! 
I have . Pull a boat and camper ? I've done that too and I wont do it again ! So why not just make the camper into a boat or a boat into a camper ? My needs are for simplicity, easy , and cheap. I dont need an extravagant expensive gas hog setup . if this works it will be perfect . Some gear locked away safely in the truck , and the other stuff safely with me on the camper boat . Pull it out of the water and directly to the camping spot and your already set up . or just camp on the water, so your already at your fishing hole for the early morning feeding frenzy 
