The genuine value of a hot tub in Cornwall is October to April. The sea is too cold to swim from late September onwards (the lidos and tidal pools excepted), and the weather drops to a temperature where evening sun on a patio is rare. A private wood-fired or electric tub on a sheltered terrace, with a glass of something, after a wind-buffeted coast path walk — that is the holiday a hot-tub cottage was designed for. In August, when the patio is the last place you'd want to be at 2pm, the tub is more often a feature in the listing photo than a feature of the week.
The variable that separates the good stock from the disappointing: privacy. A hot tub overlooked by the lane, the neighbouring property, or a holiday-park footpath is functionally useless after dark. The best examples sit in walled gardens, on private decks, or — at the top end — on clifftop terraces with nothing but sea in front of them. The next variable is heating: properly insulated tubs hold temperature overnight and reheat in 30 minutes; cheaper installations take 4 hours and burn through electricity. Listings should specify; it's worth asking.