I usually go by distance, I'm a fifth of the way there, half way there etc as I never really know how long it'll take when you're swimming so far, you don't want to have a time in your head and then discover at 5km you're behind where you thought you would be.
I personally would avoid nuts when swimming, great food but also get pesky bits in teeth and throat, taking nutrition when swimming arguably harder than when running so unless you've tried it before I'd think hard about that one. Bananas are great though, just wish I could eat them.