When your on fire, jumping in water will put it out
This is something that is already in the game. Currently, you need to move around in deep water (waist deep, not shallow water) for a few meters in order to put out the flames. Please be aware that you have to walk around and turning in a circle will not put out the flames.
IF u are already in water, then just crouching or jumping (animation that makes you fall laid horizontally only when u are on fire and jump). ... but this would weight over the server.
Would be nice to make already possible the suggestion.