Maybe it depends where they are. A crevice is a narrow space, a narrow gap, a narrow hole of some form. A fissure is a break, a tear. . So, sometimes a crevice is a fissure. Sometimes a fissure forms a crevice. . I guess if you make a wall with a small, narrow space in it to mount a hook, or something, then you have made a crevice. It is not a break, a tear so not normally considered a fissure. . If you find a rock wall in a mountain park & there is a place the wall has cracked to form a narrow opening, then the fissure in the wall forms a crevice (the space itself)

I will give a short practical answer: Use crevice. Don't use fissure unless you know exactly why you need to use it. Crevice is more general purpose, fissure comes off as more technical.

I think they are pretty much synonymous, at least in general usage.