For such big emitter as XHP70 is , you need in any way big diameter reflector.
If parabola is deep ( ratio diameter : depth = 1:1 ) as suggested you will get more narrow lighter and less pronounced corona.
Your parabola is shallower ( ratio diameter : depth = 1: 0,75-0,50 ) you will get bright spot with wider and pronounced uniformly lit corona ( what is IMO better )
But from other side your LED emitter is huge ; because of that spot will be still wide ( it is ok for surface outdoor use and on long distances 50m-200m+ ) but in water I think you will be completely blind ( in murky for sure )
in clear water in short distances too because eye will be difficult to adapt to the two extreme positions darkness and blinding with hotspot and in any way you get not more then (useful ) 30m for lighting .
Because of that fact is such (big) emitter much more reasonable drive as a video light with uniform illumination and using smaller weaker die for driving light for signaling and observing (IMO ) of course you can dim LED
but in any way you get wide concentrated beam . ( special because you have 4 separated die inside dome ) .
Key to get nice beam is ratio between size of die ( smaller is better -(a point light source) ) and surface brightens (lighter is better ) ; that's why people overdrive small dies