This is (of course!) a Scottish saying: basically the point is wishes are useless. Or more simply: "Shut up and work harder." It's that simple.
(The second part of the phrase can be anything. "If wishes were fishes we wouldn't have to work" or simply "If wishes were fishes we'd all be rich." Or it might be specific to your situation ... "If wishes were fishes we wouldn't have to drive these damn trucks for a living!")
Anyone who grows up Scottish is frequently told, "I cried when I had no shoes until I saw a man who had no feet" and "If wishes were fishes..." and other gems such as "shut up and work harder".
It is an admonishment if you are lazy. It expresses the pointless futility of "wishing" for something. It's simply saying: "if silly wishes/prayers/dreams/hopes were real, we'd all be rich. So, shut up and start working."
Note that as spagirl points out there are any number of variations of the phrase, with no connection at all to fish. Example: "if wishes were horses then beggars would ride".