Is there such a thing as a biodegradable (boat propeller shaft) grease?

I’m using a Volvo Penta Propeller Shaft grease to to lubricate and seal the propeller shaft in my boat. The grease eventually gets "squeezed" out from the various lubricated spots and ends up in the keel. The grease gets pumped out into the ocean every now and then along with any water which may end up in the keel as well.

Is there any reasonable biodegradable alternatives for shaft grease which are good – or would salt water kill the sealing properties of a biodegradable alternative?

Grease is insert into the "cup" sitting on top of the shaft, and it slowly "exits" along the "blank" shaft to the left – and then falls down.