We shipped a change to avatar mass calculation on Thursday.
Hats and other accouterments no longer contribute to avatar mass when they are equipped. The "Handle" part will report zero mass. Unequipped hats and accouterments retain their original mass values.
The intent of this change is to allow all avatars to swim upwards at the same rate. Previously, hats and other accouterments would weigh an avatar down. Multiple hats could even make it impossible to swim upwards at all.
If you have any scripts that depend on the masses of parts, they may be affected by this change.
After this change, we intend to remove the option of jumping while swimming, which we have left in place until now to prevent heavily hatted avatars from getting trapped underwater.
When we make this change, attempting to jump while underwater will impart an upward force without entering the jump state.
EDIT: point of clarification, players will still be able to jump out of the water when they're on the surface.