YC21 en F2 — two vessels sunk in 1946 and dived by charter (≈1.5 hr from Stromness); descend the anchor line between them, swim the connecting route from the working vessel’s stern to the F2’s bridge, pass a narrow swim‑through to reach the bow and gun, and note holds with ropes, a gun, workshop, bench, closet and generators; wreck penetration requires proper certification and precautions.