It's not likely to be a pleasant swim.

2 seconds sound clip from the The Bridge on the River Kwai movie soundboard.

You can hear this line at 02:05:26.769 in the Blu-ray version of the movie.

Quote context

[...]

- The problem is where to hide it.

- Our side of the river is quite obviously used by the Japanese. They'd spot it at once.

- Look. Do you see those fallen tree trunks? And the grey rock just below them on that little beach?

- Yes, sir, I see them.

- That's our key position. It's on the wrong side of the river, but it's the only hiding place within feasible distance.

- As soon as the bridge goes up whoever's there will have to swim back.

- It's not likely to be a pleasant swim.

- Sir, I was the best swimmer in my course.

- Yeah. Well, it looks like your line of country, then.

- Thank you, sir.

- Shears, you'll have to pick a spot on our side of the bank where you can cover Joyce.

- Yai will be with you, so the two of you should be able to occupy the Nips if they make any trouble for Joyce.

- Right.

[...]