There are, of course, many many bonfire and firework events in the South Hams at the start of November or at New Year (see www.southhamsevents.co.uk), but there are a number of opportunities to enjoy fireworks over the summer.
The obvious event is the display during the Dartmouth Regatta at the end of August – see clips like this on YouTube. There are many great vantage points from the quay or from the higher streets of the town. Another big local display is in July in Torbay at the end of the Paignton Festival. If you’re really lucky you might get to see the fireworks from the sea.
Plymouth - the British Fireworks Championships
The ultimate firework experience in South Hams all year, possibly the ultimate in the whole country, is the annual British Fireworks Championships in Plymouth. Six 10-minute displays spread over two nights, with the competitors aiming not just for noise and size, but with new effects, layers etc. The display is based on a breakwater in the harbour and although the best view is from Plymouth Hoe, there are many other spots from which to enjoy the spectacle, such as by the National Marine Aquarium. The official event website has a video library from 2015 here.