If you have "just found a nest with three baby swallows" then you have a Tardis* – not a boat – as swallows breed in the spring, not in midsummer (current N. Hemisphere) or midwinter (current S. Hemisphere).
"Current": 1st August.
But anyway, for those of us who are interested in swallows and not trolls:
"Swallows and their nests are fully protected under the Wildlife and Countryside Act 1981, which makes it an offence to intentionally kill, injure or take any wild bird. It is an offence to intentionally take, damage or destroy the eggs, young or nest of a swallow whilst it is being built or in use.
The Act allows for fines or prison sentences for every bird, egg or nest destroyed." ¹
Substantial numbers of swallows arrive in Britain in the spring to breed.²
You can move the boat but the swallows will die.
Thanks. We covered the boat back up and will have to deal with a very unhappy birthday girl.
Percyqfed is right. You will kill the babies if you disturb them.