I must be too stupid to understand the problem.

but why

I see two things that can be changed:

To make it easier... as you do the mental calculations in the water you either freeze when you realise you are dead, or swim when you realise you can make it!

that's redundant, and, halve is misspelled.

I'm guessing you mean something completely different, but how would anyone know what you're thinking when you don't say?