Well I don't know if this glitch has been found before but me and a couple of friends were messing around yesterday and found it. This glitch can only be seen by other people so do it with a friend at the same time. This glitch means that your boat's graphics don't react to it's movement meaning you might have the front end of the boat underweater and be driving backwards when you're driving forward. How to do it: 1. start a custom game with a 2+ players and set the weather to midnight and rainy (stormy) 2. each person bring a boat and start driving out to sea with each other. 3. as soon as you see the warning sign turn back a bit (so you don't blow up) 4. start driving over each other like ramps so the boat get's tossed up in the air. 5. if it's worked you will start seeing your friends' boats doing all sorts of weird things. Like driving vertical, sideways, backwards, upside down, front end in or almost completely underwater. Also if you cimb to the top of a water sticking out the sea a lot it will look like you're floating to the driver.
I don't have GTA 4, but it sounds like it won't be fun after a short period of tme, meh I don't know it probably is fun.