Is it normal for acrylic caulk to dissolve?
Acrylic caulks and paints, etc. are water-based. If you can clean them up easily with water than that's a pretty good sign that they are water-soluble.

Your caulk should be water-proof if fully cured. If you applied the caulking very thick or if the temperature is cool, it could take longer than the specified time to cure. Also, I don't think these are designed to be used in a situation of standing or pooling water.


  "Will silicone stick to the acrylic caulk? Can I dry it out and use silicone on top of the corner now to get me by for several years? Would a marine epoxy be a better solution in that corner?"


You should have used 100% silicone. If you can find an epoxy that is flexible, then that might also be an option. That joint should not be solid/rigid.

Since you don't want to redo it, you should wait longer to see if it is just not fully cured. If still no good, you can try silicone over the acrylic caulk to help prevent it from dissolving. However, since that is just going to be a thin layer instead of a full bead it will be hard to guarantee a good seal.