So hear me out
We have 'flutter' for 'something moving slightly and erratically through the air like a bug'
We have 'skitter' for 'something moving slightly and erratically across the ground like a bug'
We do not have an equivalent for 'moving erratically in the water'
The precedents set by the other two are
- Unique combination of two consonants
- Unique vowel
- '-tter'
Ergo, my proposal to fill this obvious gap in the English language: 'thotter'.