This Agabus species is easily recognisable by the wavy lines on it's back, as the suffix undulatus points out. The 7.5 mm long specimen on the picture above was caught in a little canalised stream. According to Foster & Friday (2011), Agabus undulatus is an inhabitant of permanent, well vegetated stagnant water in fenland, and there is a single record for a stream. Van Nieukerken (1992) mentions that the beetle is also found in canalised streams. The forementioned stream is never completely dry.
The pictures below show that the wavy line is attached to a bright sideline that finishes on the rear half of the wing cases (elytra). This feature causes the illusion that there are two separate light spots on that spot when viewed from above, and from that viewing position we may also see two pale dots on the rear, which are in effect two small vertical stripes as can be seen from aside. A closer look at the spot and lines reveals these are places where the darker colouring is not present, so it's the pale chitin we see there. Click on the "page 3 picture below to see that more clearly.
There aren't any pictures of this beetle's larvae on this site, Vainstein (1969) made drawings of the larvae. They have the general appearance of a medium sized Dytiscus larvae.
Sources:
Vainstein, B.V. (1969). "Larva of Agabus undulatus (Koch, 1837)". Original title: "Lichinka Agabus undulatus (Koch, 1837)". Biologiya Vnutrennikh yod, Informatsionne Byulleten, Akademiya Nauk SSSR, No. 3, p. 21-22, 1969. Retrieved july 2013 from http://www.dfo-mpo.gc.ca/Library/27197.pdf
More sources on the Waterbeetles bibliography.