You can do it one of two ways; take one 1/4" slice and:
Blanch it for a few seconds in boiling water, and it should sink for you.
get a rubber band and attach it to a rock (other wise it will float around and never get eaten).
Either way you’d be best to scoop out the center portion of the slice containing the seeds and jelly-like material. This will just make a mess by either rotting quickly or being messily “processed” by the fish. Take the left-overs (usually just a skin) out before it turns mushy.