[Miller]

I don't really see how not needing it is something against it being a thing.

Aquae Crests are easy to manage, sure. But that generally requires you to avoid dumping your crests into certain spells that are less than optimal for an Aquamancer to use due to said crest costs and/or being mediocre spells overall. As for the argument of 'Why not just use Crest Surge to get back to 9 immediately'. In most circumstances, this simply takes a ton of momentum investment to do and puts you at the usual risk that invocations tend to carry, 9 momentum is a very hefty cost for something that doesn't immediately benefit you damage wise.

A toggle would be a fairly healthy quality life to allow for more management of your crests rather than avoiding the majority of spells that use it because you'd lose your big damage boost from lv9. So really, I think this would add a lot more depth to the class even if it enables the crest break/idol to be able to actually use more of the spells available to Aquamancer.

TLDR; I think the suggestion is fine and I feel all worries about this somehow enabling crest break shenanigans are misplaced as the people who usually go for that kind of build generally avoid the spells that have a hefty crest cost.