Please remember that Anti-Lead Starmie can often return in the late game and act as a clean up sweeper, which makes Hydro Pump very useful to carry for later on. This allows Starmie to fulfill both purposes to great affect.
Honestly, I feel like the lead should try to focus mainly on doing its main thing: leading/taking out other leads. Having surf late-game if this manages to survive as a lead isn't bad, especially when late-game, odds are that the pokemon that you'll be facing have taken damage before, whether from sandstorm, switching in previously into rocks, or just being hit. I would much rather have a reliable late-game attack than a useful, but riskier, more mid-game attack on a lead, considering the lead probably wouldn't have recover to do well mid-game anyway. Besides, leads die. A LOT. I'd like to make my use out of a lead as much as possible, and surf seems to do that.
However, yes, hydro pump definitely does have it's merits, which is why it would be an option on a lead; it's left to debate, preference, and how one wants the Starmie to function on their team. Still, surf seems to be the superior option to me on a lead set.
One the sweeper set, yes, hydro pump is probably better; being able to get ko's that surf doesn't give you is really nice.
And this is one reason why they should have separate analyses;
1) What does it hurt to have an extra analysis?
2) Both options are feasible
3) Both options have separate functions, logic, and even movesets. One should have recover, one shouldn't. One can have rapid spin, one must have rapid spin. One prefers life orb, one needs life orb. And those aren't even all the difference.
If Nasty Plot Mixape gets a separate analysis than specially-based Mixape (which is essentially in the same situation as this; essentially the same moves with extra options on the former and Nasty Plot), then why shouldn't starmie anti-lead get a separate analysis from non-choice sweeper starmie?