Ffion Rees was leading a sightseeing boat tour on Tuesday when they saw the mammal - which she said was “very underweight”.
She said it was the first time in 27 years of leading tours that she has seen the species in British waters.
Ms Rees said: “It was an amazing sight, but it was also very sad to see it in that condition.”
She said that during the initial excitement of seeing the humpback - which can grow up to 55ft (16.7m) long and weigh up to 40 tonnes - she failed to notice how underweight it was.
She said: “It was very thin, which was quite sad, and possibly quite young. It was not in good condition at all. It was very underweight. So it could be a youngster that hasn’t done that well after leaving the mum.”
How sad - I hope it manages to find food and put on weight … 
