Latex deteriorates quickly when submerged in water. I don't think it will be harmful (think of all the rubber bands we use for attaching plants - that is latex rubber).
Although if you mold something with a latex resin, there might be plasticizer, etc. that can leech out until the thing is totally cured. I don't know of any info on which resin is safe or how long to cure things for. Although if the resin is food grade (we used some for cake decorating mold before), I would be more trusting. For peace of mind, I would just use acrylic or epoxy resins. Those we know to be safe after curing, plus they will last longer.