I have failed to ever seen any valid reasoning for disassembling a second stage underwater as there are better options and many things that can go wrong, but if it's important to you, relax because the the X650 is easy to take apart. Just pop the cover off and unscrew the retainer holding the diaphragm in place. No tools are required.
One advantage of the X650 over the S600, G250 etc. is that you would be far less likely to ever need to take the cover off underwater. It uses a long thin lever that pivots on the air barrel and there is essentially nothing to really trap a rock, etc under the lever. At least it is a lot less likely to occur on an X650 than it is on an S600 or G250. The X650 also has a huge exhaust valve and a bit more area between the spokes supporting the valve to let any gravel out.
Parts commonality is also high even though they look different. The poppet (seat carrier) used in the X650 is shorter than the "S-Wing" poppet used in the S600, G250, and S550, but it use the same replaceable seat and the o-rings sealing the poppet are the same. So the parts likely to wear and need replacement are the same.
