It may be natural to have slugs here and there since they are attracted to moist places. You can use some plants that are natural slug repellent because of their scent. If that's not enough, maybe try building a barrier around your plants to keep them away.
So you chop up the rosemary plant and add water? The slugs are still everywhere! It's been wet but not as bad the last week. I will have to try that. For now I put out beer caps. I haven't had much luck with that yet.
You could try sprinkling some diatomaceous earth around. It works great on all pests. Unfortunately, it could also harm the pollinators, so I try to limit contact by keeping it on the soil and away from the blooms.