The guide number of a strobe works like this.
GN = Distance x F-stop
For a strobe with a GN of 20 (meters ISO 100) on land, I would estimate a GN of 10 (or less) underwater. If I want to shoot at f:11 then I can be around 0.9 meters or a bit less than 3 feet away. That is why (even with strobes) the rules about get closer and closer are what they are.