What metal burns readily in water?

Caesium is fairly spectacular.

Also spelled Cesium, it is a soft silvery-gold alkali metal with a melting point of 28°C (83°F) which makes it one of the metals that are liquid at or near room temperature along with Rubidium (39°C), Francium (27°C), Mercury (-39°C), and Gallium (30°C). This element is most notably used in atomic clocks.

Cesium reacts explosively in cold water and also reacts with ice at temperatures above -116°C (157°K).

http://www.3rd1000.com/elements/Cesium.htm

Sodium, and for what happens, go to:

link http://www.theodoregray.com/PeriodicTable/Stories/011.2/

There are at least five metals that 'burn' in water. Sodium and Caesium have already been mentioned. However, Potassium, Lithium and Rubidium also burn or explode. This page looks at the reactions of the Group 1 elements - lithium, sodium, potassium, rubidium and caesium - with water. It uses these reactions to explore the trend in reactivity in Group 1. All of these metals react vigorously or even explosively with cold water. In each case, a solution of the metal hydroxide is produced together with hydrogen gas.

http://www.chemguide.co.uk/inorganic/group1/reacth2o.html

Magnesium, lithium, sodium, potassium, caesium, and rubidium are all metals that will burn and react with water.

Potassium is so reactive with water that it has to be stored in oil because it will react with the moisture in the air. This is not true of magnesium or thermite, which have to be ignited with a very hot flame.

https://www.chemguide.co.uk/inorganic/group1/reacth2o.html

Just to give you an idea how powerful these reactions are for rubidium and caesium:

https://www.youtube.com/watch?v=t2uwzNZAp-s