[Lewdcifer]

Bear in mind Aquamancer has to put in some work for those crest LVs. As it stands right now, the only skills that seem to consume crests are the following: Cold Splash, Brine Blade, Purifying Aquae, Water to Wine, Drowning Chant, and Call Storm.
I don't think it'd hurt that much to give us the option to better control our crests if we don't want to use them up on these skills.

Also, note that you're talking about an invocation for 9 momentum, and if we want to be real here, in the momentum race you're more prone to getting your ass handed back to you if you dare try that... not to mention it still runs into the same exact issue I'm trying to tackle here.

The power spike you speak of is at LV9, which is fine. You're working for that damage boost and it's not immediate. It's made easier by maining the class, or going monoclass, but those are aspects you should be rewarded for and that we shouldn't really punish here. If you want a point of reference, though, Boxer already has a similar mechanic with Schwarz Sturm giving +5% damage to all Boxer skills per LV, for a maximum of +40% at LV8 after enough skills are used, and I don't really see it getting out of hand if at all. I'd argue the same principle applies here.