I beleive this is the larva of the Drone Fly (Eristalis tenax), which I found swimming in my garden pond this morning. It was very hard to photo for it constantly kept moving around. It swims by whipping its long tail through the water, and it also uses this tail as a siphon which can extend to about 5cm long acting as a snorkel to breathe under water whilst feeding on decaying organic material. It also has stubby legs to grip rocks and soil below the water or out of the water.
It lives in still or stagnant water like ponds and ditches, and when fully grown it leaves the water to find a sheltered, drier habitat to pupate. The pupae are reddish-brown in colour and retains the long tail which makes it resemble a small rodent.
Photographs taken June 2016, garden pond, Staffordshire.