Sea enemy sometimes bounces off without killing the player

[czak]

I haven't tracked down the precise conditions of this bug yet.
It appears that when there is a corner in the sand and the sea enemy runs into this corner, it bounces away. Instead it should kill the player.

Track down the conditions; reproduce in a unit test; fix.

[czak]

In the following scenario the sea enemy in the upper left corner bounced off the wall & the sand.
It should have killed the player, but didn't.