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Considering that all major battles take place on land, and the Garicians have penalties on land, there's no point in bothering with ships. They don't require that many resources.
Added 1 minute ago
Only until a normal level, when land units can improve this.
Added 1 hour 22 minutes ago
Evaluate our balance regarding movement on the map. We propose the following: when exiting the water, there are stables where the hero needs to buy a horse; otherwise, the hero's movement is reduced by a factor of n. That is, the hero can move on land, but it's costly. The same applies to land heroes – they can move on water, but slowly. A horse, like a ship, can be summoned with a spell if you left it somewhere. If it's outside the zone, you can buy a horse at the stable, which increases movement on land. The same applies to land units – there are movement enhancers on the sea, where you can buy a ship. The presence of an artifact for movement on land or water remains necessary. If there is no artifact, the ability to move on land/water is absent.
That's it! :)
Regarding horses: Why would amphibians need them? It would be ridiculous.
It's better this way:
1) Don't take anything away from land units. Leave ships for movement on water. An artifact is required for underwater movement.
2) Amphibians need an artifact to move on land. A sphere or something else. They can swim on water without ships, but they cannot use ships. And they can surface at any point in the sea that corresponds to the underwater world. For this, they have a movement penalty on land, say 1/3 of the speed. Something like that.
And no horses!