In addition to being a standard butt plug, you can use the tunnel hole that goes all the way through the plug for lube insertion, fingers and watersports.  The clear version also makes it possible to see right into the other person.

Made of PF Blend, the Tunnel Butt Plug is safe with all lubes.  This product is not compatible with latex products.  Wash thoroughly with mild soap and water.Dimensions    Medium    3.5 inches Insertable  6.9 inches Circumference Large    4.5 inches Insertable     8.8 inches Circumference Extra Large   5 inches Insertable 10 inches Circumference