You are probably thinking about artillery, that is not what a grenade harpoon is. It is an harpoon with an exploding head. Once the harpoon have gone into the whale, it explodes and knocks the whale out. Ref: http://www.slate.com/id/2143986/
This is how modern whaling is done.

Not many scuba divers or seals around the faroe islands I think. I believe the fishing is poor these days as well, without having checked.
Ref: http://en.wikipedia.org/wiki/Faroe_Islands