Since its only happening
after surfing- dont stop surfing

Seriously though- how about getting a series of overnight tests done with a blood pressure monitor thingo,
Do the test without swimming, with swimming, with surfing, with nookie etc.
"purely for reference and research purposes" would be hard for the missus to knock back

Pretty sure most health insurance funds cough up for the monitor hire and tests as long as they are referred by your G.P.
stephen