Golduck is single-typed Water actually. Starmie is dual-typed Water/Psychic, but that's irrelevant to the simple fact its water moves hit harder than Vaporeon's.
Besides that, having a lower CP "glass cannon" is actually beneficial when trying to train an ally gym, especially if you're good at dodging which makes the HP differential less significant.
Like I said before, Vaporeon is probably the best all-round water type, but when /only/ considering offense (disregarding your ability to stay alive long enough to use said offense), there are statistically superior options.