How fun! I hope you come back and let us know about how it goes, which I'm sure will be a huge blast!!! I am so jealous. Lol. We've only done it in pools & hot tubs. So you already have a place planned out?

Hmmm... I'd probably apply it right before getting in the water, just to be sure. I'd use a silicone, thick lubricant though. Unfortunately, I don't think I can suggest one as I have yet to find a silicone lubricant that was thick enough for me on land, much less in water. Lol. Depends though. If you thin lubricant work for you, then you'll have an easier time finding one. You will be in water though, so that will also be a help for lubrication.

While we are not into group sex, we do have dive buddies who wanted to try this with each other as well (also a long together couple), and when it finally came up in conversation (as we were discussing sex on space flights, actually), we said "let's do it", so we have a place scoped out. We will be diving a wreck, in "deep" water. We know there is lots of opportunity for us to have "private time" with our partners, but still be in "emergency/help" distance from each other, on opposite sides of a large, standing ship wreck wall. Yes, we planned it out. It was harder figuring out how to get the "dry suits" to cooperate, but we got creative there too..with some old suit's and scissors. We are some crazy adventurers. We even bought "Spare Air" tanks as backup's to our Octopus "back up's" since we will be essentially out of sight of each other for a short while. We are playing it very safe.

We used ID Millenium, but again, we weren't on a deep dive like you're planning so I don't know how/if the temperature change would make a difference.

~M