My son and I were at Blue Grotto last weekend. The visibility for the first 30 feet was about 3-5 feet. The water level is high and there isn't much flow to clear the low viz area. Once you are below the green stuff, you'll need a flashlight but visibility is fine (50+ feet). The good news is we practically had the place to ourselves. However, if you're uncomfortable with low visibility, I'd go elsewhere until the water level drops.
On the other hand, Devil's Den had very nice visibility. The water level is high there as well, but the only negative impact at DD is you don't have the platforms above water level if you need to faff around a bit. If you want to go to DD, either go on a weekday or get there early on a weekend. I think they limit the number of guests and you may end up waiting several hours for entry.