The sail is needed. Not only for placing sensors, but because without it, the sub would rotate on its longitudinal axis without control.
That's why all long-traveller fishes have the upper fin (don't know the name in English
).
You could say:" Whales haven't got one". Well, whales are not fishes but mammals, some whales have it and the whales without it have to compensate this with the movements and bodyshape.
Thus, the sail is the best design at this time... removing it implicates changing the design of the whole sub... maybe even the propulsion system.
Despite this, the submarine on the photo is horrible!!