Some more information.

I want this to be a boat (something that has definite sides, as
opposed to a raft that is more flat) that will carry at least one
person (that would be me). I'm thinking something with a vaguely
canoe type shape. Power provided by the occupant via either a canoe
or kayak paddle. I want to build this in my garage and it needs to be
of a size and weight that one person can move it around and transport
it in the bed of a small pick-up truck (I have a Ford Ranger with a
bed that's roughly 4 feet wide and 6 feet long with the tailgate up,
but securing it for transport with the tailgate down shouldn't be too
much of a problem.)

I forsee using this boat in calm water conditions on a still river or
small lake. (Places that are within easy driving distance of
southwest Fort Worth, Texas, if you're familiar with the area.)

What I've figured out so far is that I can secure two bottle together
at the caps by drilling a hole through each cap, bolting the caps
together, screwing the caps back on and sealing them with something
(I'm thinking silicone caulk or possible liquid nails or something).

One of the main things I'm trying to figure out now is how to connect
more than two bottles. I don't see an easy way to connect bottles by
means other than the cap, without compromising the structural
integrity of the bottles.

One of my cubemates suggested cutting off the bottom of some of the
bottles and putting another bottle inside and then gluing (or
caulking) the seams. While this would probably solve the issue of
connecting more bottles end to end, it doesn't help me connect bottles
on the sides (which I will need to do to get more than just a long
string of bottles). So, I'm still thinking about that one. I'm
looking for something a bit more sophisticated and more durable than
duct tape or rubber bands to hold the bottles together.

Also, I don't want something that has a shell and then uses bottles
for floation. I want people to look at this boat and be able to
instantly understand that this is a boat that is completly made out of
two liter bottles (and something that holds them together and
something that seals the bottles).