Here's a brief explanation of the anchor symbol in Christianity, specifically the Anchor Cross. I found it on a website dedicated to Christian Symbolism,
www.christiansymbols.net The website has a search feature if there are symbols you come across in BBM and want to understand them better. Anyways, here's the quote
-THE ANCHOR CROSS – 1. A cross in the form of an anchor. It symbolizes hope and stability in Jesus. The anchor cross is an ancient Christian symbol, dating back to the 2nd century. It was frequently used in the catacombs to decorate the tombs of the dead. Because it was illegal to practice Christianity in the Roman Empire, symbols used in the catacombs were frequently of common objects (anchor, shepherd, dove, etc.) that could hold double meanings. In this way the symbol would not look blatantly Christian, concealing its Christian meaning from all but the informed few.
I find the connection between the use of the anchors in the catacombs and its appearance Jack's room interesting. Of course, in the case of Jack's room, the meaning is inverted. Christianity is not what needs to be kept hidden from "all but an informed few", but instead Jack and Ennis's love for each other. Maybe here it's meant to symbolize Jack's hope in Ennis, specifically Jack's hope for stability. Notice the anchor in Jack's room is also a source of light.