I need help with forces on floating objects!

robloxapp-20220516-0935147.wmv (1.1 MB)
I’m currently working on a game that depends on pseudo-physics floating mechanics. The intention is to have players able to walk around on their raft and occasionally interact with it in the water.

The “floating” (applied to both the player and the raft) is done simply with a BodyPosition instance that attempts to set its Y level to that of the water. With the BodyPosition on both player and raft, there is no component that enacts any force on the X or Z axis.

Why then does the raft, whose mass is magnitudes greater than the player, get tossed around and fly out of control so easily? I’m quickly becoming frustrated with Roblox physics!

EDIT: link to gif