Hobby Horse => AGGRIEVE
Would granting it some antisub properties be a buff? Most amphs these days surface to attack, so i somewhat doubt the claim that it means that it starts suddenly countering the totality of amph.
Flying into the range of a bunch of surfaced scallops and buoys generally sounds like a very bad idea already; meanwhile, the only combat amph that simply does not ever surface is the Duck and i'm not sure the Revenant would do much to those anyway.
(The other non-surfacing amph units are Conch, Lobster, and Djinn)
The antisub version of the weapon could also be tuned similarly to how Duck has different properties to torps and missiles - for example, Revenant in antisub mode could unleash ballistic depth charges rather than missiles. This would allow the interactions to be tuned further as needed.
An antisub-capable Revenant could be somewhat threatening to Seawolves i guess, but between Cutters and Zephyrs, if the ship AA complement can actually deal with
anything, it's that.