Yes, there is a clutch inside the gearcase to engine both Forward and Rev gears. Not a common problem with Mercury product, but sometimes it happens.

If you can, verify that the prop is not ventilating first <---coming out of the water.

Once verified, and if need be, remove the brg carrier nut located behind the prop and take out the propshaft. This gains access to the clutch...you can then check its condition.

TMD