As the school holidays begin, the public are urged to look for the creatures along the coast, as well as keeping an eye out for washed-up jellyfish.
Two campaigns hope to engage the public in unearthing more about Britain's marine wildlife.
The launch of the National Whale and Dolphin Watch comes in the same week as the annual National Jellyfish Survey.
The Marine Conservation Society (MCS) is hoping to uncover further details about the little-known habits of British jellyfish.
Large jellyfish blooms have already been reported washing up on beaches in England, Scotland, Wales, Northern Ireland and the Isle of Man and, as the UK's seas warm up during the summer, more are expected.
The moon jellyfish is the most widespread species, occurring all around the UK coast from May.
The lion's mane jellyfish has the most powerful and painful sting, though it is rarely seen south of the Irish Sea.
The MCS also received many reports of the usually rare Portuguese Man-of-War from beaches in south-west England last summer, but none have been spotted so far this year.
Peter Richardson, MCS Biodiversity Programme Manager, said: "We started receiving reports of stranded Lion's Mane jellyfish off Wales and Northern Ireland in June.
"Lion's mane jellyfish and some other species can sting, so as ever, we are encouraging holidaymakers to take part in our national jellyfish survey, but the key message is look, don't touch!"
Dolphins and whales may be more difficult to spot, but it may surprise many people to know that 28 different species of dolphin alone live off the UK coast.
Run annually by the Sea Watch Foundation, the national survey will provide a snapshot of the distribution of cetaceans (the collective term for whales, dolphins and porpoises) around the British Isles and should help scientists to understand their behaviour.
Sea Watch Foundation Sightings Officer, Gemma Veneruso said: "The more we can understand about our marine mammals, the more we can help ensure the best measures are put in place to save them."
Sightings of the Common Dolphin, Striped Dolphin, Minke Whale and Humpback Whale have all increased over the last few years.
There have been 45 sightings reported since Sunday.