For someone (actor muɴ-, CVCCV- + -um- & theme) to playfully float downstream.
Mun'annuy nan ung'ungungngan mun'ay'āyam hinan mabī'ah an danum.
The children who are playing are playfully floating downstream on the swift water.
For someone (agent; s agent maN-) to playfully float something (theme i-, -on) downstream, as a toy boat, a leaf.