The Met Office forecasts highs of 31°C in Bournemouth, Christchurch, and Poole on Monday, rising to 32°C on Tuesday and 35°C on Wednesday.
However, the RNLI has warned that water temperatures may remain “dangerously cold” despite the hot weather.
Swanage Lifeboat Station said: “It’s getting hot out there!
“And we’re all looking forward to some time on the beach, but the RNLI wants you to remember that water temperatures may remain dangerously cold.
“If you’re heading to the coast, remember to #FloatToLive if you get into trouble in the water.”
The RNLI recommends following five steps, including tilting your head back and relaxing to help you float.
An amber warning for extreme heat is in place for Monday and Tuesday.