MesserschmittMe-109 W (Source: AJ Press Monografie: Messerschmitt Me-109 Cz. 3)

In 1941 Messerschmitt was asked by the RLM to research the possibility of creating a floatplane fighter based on the 109. The company choose the F-series as its basis for this design and added floats from a Arado Ar-196. It is believed nothing came of this idea and it was abandoned. A float equipped 109 would have been very heavy and cumbersome and would have been no match for land based fighters. The same experience was made by the British with their Spitfire floatplane and the Japanese with the Zero floatplane fighter.

See the Me-109W page from my Airwar 1946 site for a humorous speculation on the Me-109W.