The largest of the water beetle families, this group consists of six subfamilies, all of which are carnivorous at both the adult and larva life stages. 

 Subfamily Copelatinae is largely tropical, with just one British representative, Copelatus haemorrhoidalis (Fabricius).  This species is 6-8mm long, maroon-red on top with red appendages.  It can be found in well-vegetated ponds and ditches, including brackish water.

 The subfamily Laccophilinae has three British members, all small (3-5mm) beetles in the genus Laccophilus.  The rarest of the group, Laccophilus poecilus Klug, was thought to be extinct in Britain until rediscovered near Lewes in 1996.  All three species have hind tarsi comprising broad segments with long lobes below, distinct from the front and mid-tarsi which have simple segmentation.  Tan-coloured with greenish hints (lost after death), they tend to inhabit the vegetated margins of large slow-flowing waterbodies.  In the field they are easily recognised in the field by their ability to jump. 

 The remaining four subfamilies correspond roughly to the small (Hydroporinae), medium (Colymbetinae and Agabinae), and large (Dytiscinae) diving beetles, and are generally coloured black, yellow, or reddish brown.  The 14 species of Dytiscinae, typified by the great diving beetle Dytiscus marginalis L. are some of the most familiar British beetles, often pictured in general guides, but there are six similar species and so species-level ID is harder than may be first thought.  Females often have grooved (sulcate) elytra, while males have circular sucker pads on the fore tarsi for mating purposes.

 The medium-sized diving beetles, 29 British species in Agabinae with a further 7 in Colymbetinae, are often the first water beetles to be encountered by the non-specialist.  In particular, Agabus bipustulatus (L.) is a common, relatively large species which can colonise all kinds of aquatic environment, and is often found in puddles and paddling pools.  Most species are usually found in vegetation and sediment around the edges of pools and rivers.

 The Hydroporinae are a large group of small beetles, with 64 British species.  They are often pioneer colonisers of new waterbodies (particularly Hygrotus species) or are found in shallow or ephemeral habitats.  This subfamily includes genus Hydroporus, a group of 28 notoriously difficult-to-ID species in Britain, which usually require dissection of the male and the use of a good reference collection for reliable ID.

This family is represented by a single species in the British Isles, the screech beetle Hygrobia hermanni (Fabricius).  A large (9mm) yellow and black water beetle, it is found in ponds and ditches, often in the soft mud where both adults and larvae are predatory.  It can stridulate loudly when disturbed, hence the common name.

Formerly part of Dytiscidae, this group has only two representatives in the British Isles.  Both species are found mainly in stagnant water in association with floating rafts of vegetation, often burrowing through mud at the bottom of the pond.  Unusually, members of this family pupate in cocoons attached to the roots of water plants.  The predatory adults can be recognised by the presence of a ‘noterid platform’, a plate between the second and third pairs of legs.

A family of 19 small (3-5mm) yellowish water beetles of slow-moving or static lowland waterbodies.  Adults are roughly elliptical, partly carnivorous, and swim clumsily by using the legs on each side alternately.  When not swimming, they can be distinguished from other water beetles by their large, flat hind coxal plates, which are used as air reservoirs.  Larvae feed on algae and duckweeds, and consequently have specialised mouthparts which distinguish them from the carnivorous larvae of other groups.

This family, with 12 British species in Britain, are most familiar as groups of small dark elliptical beetles swimming in circles on the open water of ponds and streams.  Scarcer species live amongst emergent vegetation, rather than circling openly by day.  All are carnivorous, feeding on animals caught in the surface film.  Larvae breathe through gills, rather than from the surface, which may explain the group’s predilection for deeper water bodies.

A single species in Britain, Sphaerius acaroides Waltl.  Approximately 0.6mm long, hemispherical and black, it looks very similar to a mite but walks like a beetle.  Found mainly in moss adjoining small bodies of water, mainly in Dorset but records are relatively widespread.  Historically regarded as scarce, but its small size and cryptic habits mean it is likely to be overlooked.