As noted by DoStuffZ, sound travels faster in water than air. The problem with underwater speaking is that we (human airbreather) need to exhale oxygen to speak, which forms bubbles and alter the sound. If you hum, you'll be heard quite well, so you can try to communicate this way.
I never heard of any official description of D&D / Pathfinder languages, so feel free to houserule that aquan is indeed a way of speaking clearly underwater even for non-amphibious creatures (by humming or very short syllables that wouldn't be altered too much by bubbling).
If you're looking about rules, note than D&D3.5 aquatic elves live underwater and do not speak aquan. Aquatic druid doesn't gain aquan either.
This link to Paizo forums also make a couple of points :
I found this in stormwrack I'm still looking, but this might shed some light. It's for a Bard: "Bardic Music: A bard performing underwater is
restricted in what sorts of Perform checks she can make.
Perform (sing) is impossible unless the bard can breathe
water, whether naturally or by virtue of a spell. Oratory
is similarly ineffective."
Spellcasting Underwater: Casting spells while submerged can be difficult for those who cannot breathe underwater. A creature that cannot breathe water must make a concentration check (DC 15 + spell level) to cast a spell underwater (this is in addition to the caster level check to successfully cast a fire spell underwater). Creatures that can breathe water are unaffected and can cast spells normally. Some spells might function differently underwater, subject to GM discretion.
This would means that speaking (and even casting spells with verbal component, though it says nothing about spell with language subtype) isn't a problem as long as you can breathe.