The ornamental design for a monofinswimmingdevice, as shown and described.
1. Monofin swimming device
2. Monofin
1.1 : Perspective
1.2 : Perspective
1.3 : Top
1.4 : Bottom
1.5 : Front
1.6 : Back
1.7 : Right
1.8 : Left
2.1 : Perspective
2.2 : Perspective
2.3 : Top
2.4 : Bottom
2.5 : Front
2.6 : Back
2.7 : Right
2.8 : Left
A monofin swimming device as depicted in the reproductions; the depicted monofin swimming device can be attached to a user's feet to help propel the user through the water while swimming.; 2. A monofin as depicted in the reproductions; the depicted monofin can be used as an insert to a swimming device, which then can be attached to a user's feet to help propel the user through the water while swimming.