Check out the viperine water snake.
Distribution: Spain, Portugal and North Africa.
Habitat: Most habitats, excluding dense woodland or desert, even in mountain streams.
Max. length: 500-600mm SVL (600-700mm TL).
Reproductive strategy: Oviparous with clutches of 4-20 eggs.
The viperine watersnake is the most viper-like of the European watersnakes, both in looks and behaviour, and it is frequently killed by persons who confuse it with a venomous snake. I have since found specimens in Portugal in even cold mountain streams so it appears to be plentiful. I once taught a female traveling companion to catch snakes and she brought me 13 viperine watersnakes in under one hour. I had to ask her to stop.
Sources for more information:
Arnold E.N. & J.A. Burton 1978, A Field Guide to the Reptiles and Amphibians of Britain and Europe. Collins. 272pp.
Javier L. & B. Escriva 1987, La Guia de Incafo de los Anfibios y Reptiles del las Peninsiula Iberica, Islas Baleares y Canarias. Incafo. 694pp.
Malkmus R. 2004, Amphibians and Reptiles of Portugal, Madeira and the Azores-Archipelago. Koeltz. 447pp.
Steward J.W. 1971, The Snakes of Europe. David & Charles. 238pp.