Did his helmet act as an anchor?

Police officer drowns after jumping into river to chase student.

TAKAHASHI, Okayama -- A police officer was found drowned at the bottom of a
river on Wednesday morning after he jumped into the water the previous evening
to chase a university student, police said.

[snip]

Watanabe was found on Wednesday morning some 450 meters downstream from
where he jumped into the river.

Watanabe was wearing a police uniform and helmet when he jumped into
the river.