I'm bringing forward a more mathematical observation to this discussion, warning you guys now.
When deciding between two moves I generally like to ask myself: on average, how many times to I have to click this move to secure a KO? With a 100% accurate move like Surf, the answer is simple: if it kills something in 3 hits, you click Surf 3 times, if its an OHKO, 1 time, etc. Now when looking at Hydro Pump, the answer is a little different, since now we have to factor in the chance of one (or more) miss(es). I've discovered that on average, a Hydro Pump that OHKOs if it hits takes an average of 1.25 clicks to KO the opponent, or, if we consider accuracy as a fraction of 1, the inverse of the accuracy, since 0.8^(-1)=1.25. When it takes more than 1 hit, you multiply the result of the OHKO by the number of hits you need. So, for a 2HKO you need 2*1.25=2.5 clicks to KO on average, 3 is 3.75, so on and so forth.
Let's assume Hydro Pump always KOs in one less hit than Surf. Obviously this isn't always the case but it makes explaining this part a little easier. This would mean that Hydro Pump is generally better unless Hydro Pump has to hit 4 times (which given that it is likely used with rain means this situation is rare), or when Surf OHKOs (since you have to click a move at least once to kill with it). That said, when this is taken across all of OU there will likely be cases where both Hydro Pump and Surf would be favorable over the other, so ultimately the "correct" choice takes advantage of the more common case.
The case with Hydro Pump and Surf can also be applied to other types with adjusted accuracies but similar power differences, like Flamethrower and Fire Blast, or Aura Sphere (not HP Fighting) and Focus Blast. Just change the clicks to account for the accuracy change (about 1.18 for Fire Blast or 1.43 for Focus Blast).
When deciding between two moves I generally like to ask myself: on average, how many times to I have to click this move to secure a KO? With a 100% accurate move like Surf, the answer is simple: if it kills something in 3 hits, you click Surf 3 times, if its an OHKO, 1 time, etc. Now when looking at Hydro Pump, the answer is a little different, since now we have to factor in the chance of one (or more) miss(es). I've discovered that on average, a Hydro Pump that OHKOs if it hits takes an average of 1.25 clicks to KO the opponent, or, if we consider accuracy as a fraction of 1, the inverse of the accuracy, since 0.8^(-1)=1.25. When it takes more than 1 hit, you multiply the result of the OHKO by the number of hits you need. So, for a 2HKO you need 2*1.25=2.5 clicks to KO on average, 3 is 3.75, so on and so forth.
Let's assume Hydro Pump always KOs in one less hit than Surf. Obviously this isn't always the case but it makes explaining this part a little easier. This would mean that Hydro Pump is generally better unless Hydro Pump has to hit 4 times (which given that it is likely used with rain means this situation is rare), or when Surf OHKOs (since you have to click a move at least once to kill with it). That said, when this is taken across all of OU there will likely be cases where both Hydro Pump and Surf would be favorable over the other, so ultimately the "correct" choice takes advantage of the more common case.
The case with Hydro Pump and Surf can also be applied to other types with adjusted accuracies but similar power differences, like Flamethrower and Fire Blast, or Aura Sphere (not HP Fighting) and Focus Blast. Just change the clicks to account for the accuracy change (about 1.18 for Fire Blast or 1.43 for Focus Blast).