What a fantastic place you are off to visit, I love it and have been there many times BUT not with a boat. However the local dive shop in Stromness is run by two great people Leigh and Dougie. Dougie is a local dive boat skipper and knows the flow like the back of his hand. I am sure they would be very helpfull if you give them a ring.
(However if you are intending to dive then a word of warning, diving out of a rib can be met by the locals by ZERO co-operation as the area relies heavily on divers using the local dive boat charters. If you are not diving then make sure you tell them if you phone that you are only RIBing)
Leigh & Dougie
The Diving Cellar, 4 Victoria Street,
Stromness, Orkney, KW16 3AA
Telephone (0044) (0)1856 850055
Fax (0044) (0)1856 850395