3 mm in Millbrook?? You're a braver man than me!
I won't set foot in there without a 7 mm.
The great thing about Millbrook is its proximity to Northern VA residents. I plan on exploring it a little more this fall once my drysuit comes in. I hope to never dive it wet again...
Of interest to locals - I did hear that Juturna Springs got their zoning issues resolved and should be reopening in 2020. For us in NoVA thats a much better drive than Lake Phoenix (1 hour from my house for Juturna, 3 hours for Phoenix), and honestly that site once set up might have more potential. I went there twice before they shut down and I really enjoyed it. Everything was kind of sparse as they had just opened but the platforms and walkways that were in place already were well thought out and well built. Looking forward to it.