What you Need
A 500ml bottle
Bicarbonate of soda
The use of a drill
What to do
You want to build a nozzle into the lid of the bottle, to do this first drill a hole in the top (or get someone with a drill to make the hole for you.
You then need to hold the straw into the cap against some pressure, so put some plasticine, or blue tack around the straw, inside and outside the cap
The plasticine on the inside is most important for making a seal, as the pressure will tend to push it against the straw, on the outside it is mostly acting as support and weight to make sure the straw is below the liquid level inside the bottle.
Fill the bottle a third full of water and test that it floats fairly level but with the neck of the bottle lower than the base.
If you are using strong vinegar dilute it with it's own volume of water.
Now fill the bottle a third full of vinegar.
Very quickly add 1/2 a teaspoon of bicarbonate of soda, put the lid on, and put the bottle into a bath or a sink.
What may happen
You should find that a jet of vinegary water is shot out of the straw and the bottle is pushed through the water.
Why does it happen?
Bicarbonate of soda (Sodium hydrogen carbonate) has carbon-dioxide gas locked up inside its structure, if it reacts with an acid such as Vinegar (acetic acid) it releases carbon-dioxide gas
Sodium Hydrogen Carbonate + Acetic Acid -> Sodium Acetate + Water + Carbon-dioxide
This carbon-dioxide is a gas so has a much larger volume than the vinegar and Bicarbonate of soda so it bubbles up and it pushes outwards, building up the pressure
This pressure pushes vinegar and water out of the straw
The equal and opposite reaction pushes the boat along, using the same principle as a jet or rocket.