I wasn't envisioning a promotion, just a utility specialist that could fill in to aide you or hinder an enemy. I like the Water Worker suggestion though.
My initial concept was that since there were so many other suggestions for wanting a way to slow down enemy ships I thought this would be a good way to add something to the game that would provide that.
Maybe if there was a Water Worker promotion, it could do either:
A. an actual 50% reduction or increase per boat's speed in the flow lane (so it stacks as a multiplier to enhanced speed versus just adding or subtracting the .5 - it's the same thing for all boats moving at 1, but things that go faster than that would become more drastically affected), and add an additional flow lane that must go to a second center.
or B. add an additional flow lane and leave the option open that both lanes could be routed to the same center to give a total of +1 / -1 speed in the lane and speed up the option to update flow lane to every 4 hours. That would mean that any specialist laden boat without speed & any normal boat in that lane without speed enhancement from Admiral would just get stuck in place though.
I could also get on board with a localized effect for the promoted specialist - enabling a sub he was on to move at +50% speed-stackable with other speed perks, finally giving a local speed effect that would combo with other speed specialists. The rationale being, that although multiple speed specialists on a boat can't provide redundant boat movement efficiency, the Water Worker actually has altered the flow around a sub he's on and that works with the other speed enhancements other specialists provide.