I see you're contemplating changing (just) tanks under water. A good way to wreck the regulator, and possibly end up with it failing during the dive. Why not change tank+regulator, having them assembled above water, or even the complete scuba?
Where will the attempt be - inside or outside the reef? If inside at Hol Chan, remember the severe currents four times a day.
I have done 5 hour dives around here (San Pedro) and I ended them through boredom.
The plan is to be diving an OTS Full face mask with communication unit. I'm looking at using a gas block with a sidemount harness so when one tank runs low, it would be a matter of changing the valve to the full tank and swapping out the low tank. With this setup, the first stage would be safe from the saltwater as that part would already be connected to the tank. This setup should only allow a small amount of seawater into the second stage through the quick disconnects on the gas block. There will also be a complete gear set staged on a line ready for use if the need should arise.
