Liquids can be placed on the back of rapidly extending pistons while you are in a boat..

How to reproduce:

1) Place down a piston facing you.

2) Connect it to a fast redstone clock (but make sure there is air directly behind the piston).

3) Place a boat down in front of the piston and enter.

4) Aim at the middle of the piston, hold a water.lava bucket in your hand and hold right click (or whatever key you use to place blocks).