Sealant for external outlet
Actually 100% silicone isn't a great caulk in my book. It's hard to get off later and typically isn't UV resistant.

What you want is a solid outdoor oil-based (mineral spirit cleanup) caulk (sometimes called Window/Siding). I really like Quad, but anything made for outdoors will do (even some latex products).

If I were you, I'd also replace the cover. It's looking weathered. Once you've caulked it, you'll be less likely to replace it in the near future. I would highly recommend an in-use (bubble type) cover.