If you're Paul Semmens
you jump in and go meet the critter face to face.
No, Paul is not mad, he's a just a "shark guy". There's no rhyme or reason for wanting to dive with a 30 foot Basking shark except for the joy of the "moment":
Photographer Paul Semmens captured images of the shark from the ferry The Scillonian which runs between Penzance and the Isles of Scilly as it fed close to the surface on plankton.
The size of the creature is revealed by the distance from the dorsal fin to the tail fin which is half its total length.
"It was around seven metres - over 30-ft long - a big one - but they do get bigger," Mr Semmens said. "June is often a good month to see basking sharks because the sea has warmed up and the algal blooms which they feed on are growing.
"I've seen over 20 of them off Cornwall recently - which is an average number. A couple of years ago there were over 300 spotted in one day which was a record."