Men in wetsuits can fight a few brambles with a handheld light. The route is NE to E out of The Chequers down to the sea wall. I'm not sure what the Blackwater river policy is on motorised boats/watersports but in any event the noise could potentially alert others. A windsurfer is very quiet other than a bit of noise from the sail.
You're overegging the pudding, as usual. If JB did go by the BLUE sea wall route, he would have chosen to keep himself dry by cycling or walking, not fiddle-faddle about with a surfboard. For starters, he'd have to carry a large, awkward object from the pub hiding place to the sea wall, then back again on the return journey. And what would happen if the wind was blowing in the wrong direction, or he became becalmed... he's stymied!
Nope, sorry... I'm sticking with the shortest RED route, travelling by bike via the farm track at the back of WHF.... as he confided to his girlfriend.