SQL - Identify where keys are being used to build SQL Statement

I attempted to duplicate the issue I am experiencing.  I have 3 tables (Ship, Items, ItemsShip).  I am trying to resolve a logic issue and write a SQL that will get me the data that I'm looking for.

Item table relates to Ship table by "ShipID".  
ItemShip table relates to Item table by "ItemID" and Ship table by "ShipID".

The logic was written with .NET with regards to how the data is inserted.  I don't understand why ItemShip was created considering that Items table contains a "ShipID" field.

Regardless, I am getting data between Ship and Items.  However, I need to check to see if there is data in ItemsShipped which should be joined.

Should I just do something like

FROM Ship S
INNER JOIN Item I on I.ShipID = S.ShipID
INNER JOIN ItemShip IS ON IS.Item = I.Item
                                           AND IS.Ship = S.SHip

CREATE TABLE #SHIP(
		ShipID	INT,
		Name	VARCHAR(20)
)

CREATE TABLE #ITEM(
		ItemsID	INT,
		ShipID	INT,
		Name	VARCHAR(20)
)

CREATE TABLE #ITEMSSHIP(
		ItemsShipID	INT,
		ItemsID	INT,
		ShipID	INT
)

INSERT INTO #SHIP
(ShipID, Name)
VALUES
(1, 'Ship #1'),
(2, 'Ship #2'),
(3, 'Ship #3'),
(4, 'Ship #4'),
(5, 'Ship #5')

INSERT INTO #ITEM
(ItemsID, ShipID, Name)
VALUES
(1, 1, 'Item #1'),
(2, 4, 'Item #4'),
(3, 5, 'Item #5')

INSERT INTO #ITEMSSHIP
(ItemsShipID, ItemsID, ShipID)
VALUES
(1, 1, 1),
(2, 2, 4),
(3, 3, 5)

SELECT * FROM #SHIP
SELECT * FROM #ITEM
SELECT * FROM #ITEMSSHIP

DROP TABLE #ITEM
DROP TABLE #ITEMSSHIP
DROP TABLE #SHIP

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