And I vote the other way.
Aquatic animals clearing out kelp is a bad thing, for multiple reasons. First off, it's unrealistic (kelp generally has more animals, they don't clear it out to the extent that would be required for this),
But, the principle illusion of "realism" is still there - animals eat. Realism-in-detail is something we never see in fiction, television or gaming. Ever see a bathroom on the Enterprise?
Having an animal be able to eat a naturally occurring plant (spawning/respawning troublesome kelp) is perfectly within reason and doesn't break "suspension of disbelief" any more than fireball throwing mages and centaurs. What does a sea serpent eat in real life anyway? Who say's they can't gobble up an entire kelp forest?
but second and more importantly, it would result in the oceans being cleared of the kelp that is placed there by the map.
If kelp has a spawn rate, wouldn't the game replace the kelp over time? Or, can it only spawn once per tile? Here, it's the mechanics I don't know which could be the barrier.
The point of them being able to automatically eat the kelp is to help keep kelp growth down in areas the player is not in. As kelp growth increases within the player's territory (because animal units will not cross into it) there becomes some incentive and a bonus for capturing sea creatures. Greater expansion means a little more unrestrained kelp growth. With greater expansion comes the predicted greater ability of a player with seaside cities effected by kelp to hunt sea creatures. In other words, if kelp is a problem for the player, they will have a means to deal with it simply due to the circumstances surrounding why it is a problem. (Unless territories are so large that few, if any, creatures will spawn. But, your mechanics seem to still allow for spawns and I would imagine it wouldn't be too much of an issue in the late-game.) In essence, it's also playing on sort of a environmentalism type of mechanic - There are consequences to sea-side expansion that one must allow "natural" forces to help control.
The only true problem with this suggestion has to do with the AI and the ever-present problem of burdening the game with yet more calculations to do every turn. The AI, AFAIK, isn't capable of making "judgement calls" that would determine its use of sea creatures in this way in a sensible fashion. While I've seen AI ships with the promotion, I've never seen a squadron of them hunting for sea creatures either. "Chance" might not give them enough opportunity and, even if it did, they might not know what to do with them to begin with. (I suppose whatever is used for the Work Boats could be cobbled into a promotion for sea creatures so the AI would know what to do with them. Where they decided to Feed might be ranked on commerce points/trade-routes/etc.)
Anyway, it's just a mechanic suggestion to add some additional flavor to captured sea creatures. There was no intent to make it anything obtrusive in its impact. It's a "tweak" that really shouldn't be constructed to have any great impact on the player, regardless of their circumstances. But, it could add some fun for those who pursue it. Otherwise, the majority of what the player will use them for will be simply more military units. (Note: I've never been able to build a Sea Serpent's special building. I haven't looked into whether that's a bug or I'm just not doing it right.)
Oh, an additional note: Sea Turtles are a bit too strong, IMO. The fiery breath attack along with a meteor swarm for a unit that a Frigate straight out of the dock can easily capture is a but much. True, you have to find them. But, once you do, no seaside city is safe. They're slow, but not unmanageably so. However, they're really miniature Arcane Barges in offensive capability. A couple of Sea Turtles with an escort to protect them from attack will quickly reduce a seaside city. Sure, they should be more powerful than a serpent. But, either the Breath Attack or the Meteor Swarm, but not both, would be a little better, IMO. Lowering the strength of either or both might also be an acceptable fix.