A cable guide includes an upper face and a lower face. The lower face has a generally convex form so as to lie in contact with the flow channel when fitted in the flow channel. The upper face has a generally concave form and is shaped to minimize disturbance of the flow when fitted in the flow channel. A passage is defined between the lower face and the flow channel having an entrance opening at a first end of the cable guide and exit openings on the sides of the cable guide towards a second end. By utilizing a cable guide, disruption to the flow within the channel by the entrance or exit of the cable is minimized.