The second photo from the top shows a "rubber" moisture seal. When the back it put back in place, it will seal out moisture once again.

Is there not a second battery in a chest strap sensor? I do not have a Polar HRM, but I believe the chest strap battery is more difficult to change.

Rather than using wax to seal an HRM not only from rain, but also from perspiration or humidity, silicone gasket sealant woud be a good choice.