Bac Lava: There are many places to enter the sea in this archipelago of 300 islands, of which only six are inhabited. You're likely to start on St Mary's so head to a sheltered beach and take your first dip in the clear, cool Atlantic water from the lea of the island.
Since there are so few people it's relatively easy to find a secluded spot to abandon the last stitches of your cossie without fear of offence; on the other hand you might prefer a wetsuit to allow you to spend longer enjoying the rugged scenery from the water.
There is great snorkelling at Green Bay where you can look down on weed-encrusted stone walls of drowned valleys, lost as the islands have sunk slowly into the sea. The book Wild Swim by Kate Rew, details specific crossings; and the specialist swim-holiday company Swimtrek have week tours swimming in the Scilly isles, guiding you on an island-hopping adventure as the mood and weather takes it.