OK, update. I went back and searched all along the stream to see if it had floated for a while and got snagged against the bank by the current. No luck.
I looked into the wide area of deep, boggy fast flowing water where I would have to search - and realised it was nigh-on impossible. even for a scuba diver, the visibility was lousy, and it could be anywhere. So, whilst I took waterproof sandals and a change of clothing - I thought better of it.
I was very pessimistic about claiming on Care Refresh as someone above posted that if you can't produce the dead Mavic body, you can't claim. I thought of a Credit Card protection claim - but then remembered I'd bought it on a Debit Card - otherwise that would have been a good approach; most offer 90 day protection against loss, defect or damage on ANY item bought with the card.
So, with little hope I kicked of a Chat with DJI Support - as follows:
Me: "Hi. I bought a Mavic Pro a few weeks ago with Care Refresh. The other day it crashed into a tree and then fell into a river where it was lost. I have witnesses and photographs. Am I covered?"
Brian: "Yes"
Hooray! He is sending over the forms now via email.
My opinion of DJI and Care Refresh very much enhanced. Phew....