Is it possible to build a boat out of LEGO bricks? I appreciate you couldn't make it watertight but would a LEGO boat float by itself due to the buoyancy of the bricks themselves?
Pictures of any home-made floating constructions would be really interesting.
The plastic is a bit heavier than water and should sink, however in practice I have experienced that 2x4 bricks (and the like) have a tendency to trap air inside them and actually float rather well. It probably isn't failsafe, however for shorter periods of time (like a half to a full hour) would probably work fine. (I haven't actually tested the time it could stay afloat, I'm trying it out now. When my small mini-boat has sunk, I will add the time here.)
Update:
24 hours have passed, it is still floating.
It sunk about 1mm compared to the beginning, but nothing of importance. However while I did push it around from time to time, it was still in still water and stationary most of the time. How it will perform during play is unknown.
Notice how the internal structure of the bricks causes the air to be trapped inside them.
A few years back there was this solid single-piece hulled police boat which would float. 
There was also a small red boat (pre-2000) that came with part of a pirate set (I believe, anyway) 
As for building a boat from scratch...no. It tended to capsise then sink (somewhat impressively). Place 2x 4x2 bricks on another and place in water, they only float on their side. I expect this is down to a center of gravity but I'm not a sciencey type so I can't confirm for sure.
I had a Police-Boat when I was a child that had a one-piece-body, so it was watertight and able to float:
There are some other LEGO-boats with similar bodies (even some bigger ones) that should also be able to float.
There is another not mentioned way to achieve flotation. My kid discovered it today with his "water bat": surface tension. The long flat wings did the trick : ) It is not exactly due to the buoyancy of the bricks themselves as wanted in the question, but might help as a complement.
For more on the Physics underlying: Surface tension
In addition of the various boat hulls, you can use pneumatic tanks for extra buoyancy.
If you want to motorize your boat, keep in mind that means extra weight, which has to be countered in some way.
My kids had Captain Redbeard's Pirate Ship - it certainly floated in the bathtub!
I believe many of the other Lego pirate ships could float, particularly if they had molded single piece hulls - see this list.
My children play with the lego fishing boat (see picture) in our bath tube. This design does have a serious flaw (imo). Even if there are very small water waves in our bath tube, water enters the boat which then becomes to heavy and sinks. So if you are planning to built your own lego boat i would make sure the sides are high enough to prevent water from entering the boat. Or if you are really clever make a water drainage system like normal boats have.
Lego fishing boat 4642
You also asked for some examples. Found this simple but effective example online.
For a very small "boat" - something like a 4x8 plate with 1x8 and 1x2 brick sides it might stay afloat for a short amount of time before the leakage takes it down. For larger stuff, LEGO is just too heavy when you get in to building the necessary structure for a real boat.
There were some sealed-hull boat sets I remember from childhood that were designed to float, so using the special parts from them would make it possible.
Already a correct answer accepted, but... the original question seems to be "do Lego bricks float by themselves?"
Most Lego bricks are made of ABS plastic which has a specific gravity greater than 1 (but only slightly). So, they are heavier than water and will not float naturally. You can get small bricks to float initially since small bubbles cling to their surface, but they will all sink eventually.
Since ABS isn't affected by salt water - it might be possible to increase the salinity of the water to a point where the bricks naturally float (would need to be higher concentration than sea water...)
Yes, if built carefully. When I was a kid I made a number of lego boats to play with in the tub, and later in the pool. The main thing I remember was to make the hull upside down and out of at least two interlocking layers. Lining the inside with plate also helped.
I'd have to say that in general, only the parts that are designed to float will do so reliably. You may be able to make some sort of raft by inverting several large bricks (studs down).
I haven't tried Lego bricks in water (other than a police boat set), however, when using Duplo's in the bath tub we were only able to make submerged structures. The water easily seeps into the bottoms of the bricks and causes them to sink.
Yeah they float but make the hull wide not narrow as it will stay up right if you make the hull wide. Narrow means top heavy. I first started making LEGO boats now I make home built boats. And no need to be water tight for LEGO just get more surface area. Who knows you might even be able to make a cruise ship that floats.
Styrofoam bits trapped in brick-built chambers might work Ok to keep structures afloat.
Yes, just as long as the bottom is able to displace the boats weight onto the water.. Just like how a Polar Bear can walk on thin ice; displacement.
