Are the relays on the Mercury boards wet or dry?

The relays are dry contacts.

A dry contact output on a device like a relay means that the contacts have no voltage or potential difference across the two contacts. The contacts simply provide an open or closed state to another device.

A wet contact output on a device like a relay means that when the relay is activated, the relay output contacts supply a voltage to whatever device is connected to the relay output contacts, say for example a light or motor.