Question: Loch Shiel (Scottish Gaelic: Loch Seile) is a 17+1/2 mi freshwater loch, 120 m (393 ft) deep, situated 12.4 miles west of Fort William in Lochaber, Highland, Scotland, at the south west it ends  the 4 km River Shiel drains to the sea in Loch Moidart near Castle Tioram, Castle Tioram ( ) (Scottish Gaelic: "Caisteal Tioram" , meaning "dry castle") is a ruined castle that sits on which tidal island in Loch Moidart, Lochaber, Highland, Scotland?

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Loch Treig: Loch Treig (Scottish Gaelic: Loch Trèig, meaning "loch of death") is a 9 km freshwater loch situated in a steep-sided glen 20 km east of Fort William, in Lochaber, Highland, Scotland.  While there are no roads alongside the loch, the West Highland Line follows its eastern bank.

Loch Shiel: Loch Shiel (Scottish Gaelic: Loch Seile) is a 17+1/2 mi freshwater loch, 120 m (393 ft) deep, situated 12.4 miles west of Fort William in Lochaber, Highland, Scotland.  Its nature changes considerably along its length, being deep and enclosed by mountains in the north east and shallow surrounded by bog and rough pasture in the south west, from which end the 4 km River Shiel drains to the sea in Loch Moidart near Castle Tioram.

The Rough Bounds: The Rough Bounds (Scottish Gaelic: "Na Garbh Chriochan" ), in the Scottish Highlands, is the area of West Inverness-shire bounded by Loch Hourn, Loch Shiel, and Loch Moidart, consisting of the districts of Knoydart, North Morar, Arisaig and Moidart.  The area is famous for its wildness and inaccessibility and remains very sparsely populated.

Loch Ailort: Loch Ailort ( ; Scottish Gaelic: Loch Ailleart) is a sea loch in Morar, Lochaber, Highland, Scotland.  Loch Ailort is a shallow V shaped loch, with the small Ardnish Peninsula on the north side, and the large southwest facing Moidart Peninsula to the south.  Loch Ailort is bounded in the northeast by the settlement of the same name, Lochailort and in the west, opening out into the Sound of Arisaig.  To the north of the loch lies Loch Nan Uamh.

Seileag: Seileag or An t-Seileag is the loch monster which is said to inhabit Loch Shiel in Scotland.  The name may be a pun on "Seile " (Loch Shiel) and "sìolag ", a term for slender fish and eels in Scottish Gaelic.

Castle Tioram: Castle Tioram ( ) (Scottish Gaelic: "Caisteal Tioram" , meaning "dry castle") is a ruined castle that sits on the tidal island Eilean Tioram in Loch Moidart, Lochaber, Highland, Scotland.  It is located west of Acharacle, approximately 80 km from Fort William.  Though hidden from the sea, the castle controls access to Loch Shiel.  It is also known to the locals as "Dorlin Castle".  The castle is a scheduled monument.

Moidart: Moidart ( , Scottish Gaelic: Mùideart ] ) is part of the remote and isolated area of Scotland, west of Fort William, known as the Rough Bounds.  Moidart itself is almost surrounded by bodies of water : Loch Shiel cuts off the eastern boundary of the district (along a south-south-west to north-north-east line), and continues along part of the southern edge; the remainder of the southern edge is cut off by Loch Moidart; the north is cut off by Loch Morar and Loch Ailort.

Loch Moidart: Loch Moidart is a sea loch (sea inlet) in the district of Moidart in Highland, Scotland.  It is on the west coast of Scotland, and runs about 8 km (5 miles) eastward from the sea.  It is connected to the sea by two narrow channels which are separated by the island of Eilean Shona, which in turn is nearly split in two by a narrow isthmus.  The River Moidart enters the loch at its head, near the hamlet of Ardmolich ("Àird Molach").

Loch Morar: Loch Morar (Scottish Gaelic: Loch Mhòrair) is a freshwater loch in Lochaber, Highland, Scotland.  It is the fifth-largest loch by surface area in Scotland, at 26.7 km2 , and the deepest freshwater body in the British Isles with a maximum depth of 1017 ft .  The loch was created by glacial action around 10,000 years ago, and has a surface elevation of 9 m above sea level.  It separates the traditional district of North Morar (which contains the village of Morar), from Arisaig and Moidart

Loch Garry: Loch Garry, (Scottish Gaelic: Loch Garraidh), is a freshwater Loch 25 km north of Fort William, Lochaber, Scotland.  Loch Garry is 11 km long and 50 m deep.  It is fed by waters from Loch Quoich 10 km upstream on the River Garry, and drains into Loch Oich in the Great Glen just 5 km downstream.  Loch Garry is much photographed from the A87 for its romantic setting and also because a quirk of perspective makes it appear like a map of Scotland.

Supporting paragraphs: Loch Shiel Castle Tioram
Answer: Eilean Tioram