Georuku Why give it permanent armour instead of just increasing the health? Also it sounds like a way to turn a Lance into a discount Lucifer, and Lance doesn't really need a buff. @skigear is right in that any units balanced around being slow and weak would get much stronger with such a building. Things like the aforementioned Lance as well as any other artillery unit. It's also basically a straight up buff to Emissary, Tremor, and Fencer, all of which already had to stop to fire in the first place, so all this building would do would just give them a straight up health buff.
FOURTY_TON the fundamental concept would be cancerous, so it would need to be useless in order for there to be any enjoyment in sea. So it sounds like a horrible idea.
The problem is that there's a grand total of 11 units (Dagger, Claymore, Raven, Duck, Scallop, Archer, Hunter, Seawolf, Siren, Ultimatum, Detriment) that can reliably attack underwater targets, so in order for such a turret to be killable at least one of those would have to outrange it, and that unit can't be Detriment. So you're looking at a turret with less range than Stardust, which would make the turret borderline useless at actually defending.