So long as it does not rely on a pea like the old refs' whistles, it should be fine. Go swimming and test it. I used to take my old dive whistle on hikes, I knew it would work in a dowpour.
Batteries. Not rechrgeable. Buy new ones annually. Put a tickler in your diary.
Condoms are useful. They can hold fresh water.
Do you have one of those thermal wraps, like tinfoil, fold down to the size of a cigarette packet? No weight at all, easy to carry. Even in the tropics, at night in a typhoon you can lose a lot of body heat.
All of the above, a few grammes only. They'll fit in a side pocket on your rucksack.