I'd select colour & take the shot.

Then I'd save his life and rescue the man from being swept away & drowning.

Hey, I'm no New Labour Blair worshipper, in fact I think he's done the UK a great disservice along the lines of Neville Chamberlain 65 or so years ago, but history will be the judge of that.

But, the guy is in trouble and he needs to be rescued.

Besides a photo of you rescuing Blair from certain death will be received by just about everyone far better than if you had taken an "about to drown" shot.

No-one will thank you for being there and doing nothing!

(Thats because some might have wanted you to have rescued him but others will have wanted you to have pushed him under... )

...and in any case, regardless of all the above, rescuing the man is simply the right thing to do regardless of who he is or what he has done.