I am on my 3rd year of Ultra 2.
I had an interesting experience in Northland - boat was cleaned by divers in Westhaven. One hour later I took it to Bay of Islands, it sat for 3 days on a mooring in Russell, by which time it was covered in a thick layer of slime.
Before coming back to Auckland, (2 weeks ago), we wiped the hull and keel with a sponge, (I now know why I pay divers to do this, but that's another story), it's now been sitting in Westhaven for 2 weeks and is pretty much in the same state that we left BoI in....
There's definitely something in the water up there that