Any heavy duty mechanics on here? Oil leak in my boat engine, pic included.

[Redlyne_mr2]

So one of the engines on my boat has developed a small oil leak. I know nothing about Diesel engines. It appears to be coming out of a rubber cap that's painted white like the rest of the engine. Can anyone tell me what this is and why it might be leaking?

Thanks
Ryan

[Redlyne_mr2]

Again ..
http://i42.photobucket.com/albums/e306/Redlyne/image_zps0d70cc50.jpg

[R154]

John Deere or Cummins? Looks like a JD. PM me.

[Redlyne_mr2]

Sorry should have posted..
Volvo Penta star