How do disembarkment-battles work?

It simple. Just move your naval unit on the ground and they disembark. It will takes 3 hours on own land and 4.5 on enemy land.

While disembarking they are considered on sea so with reduced strength.

Disembarking units are considered naval units so the damage vs naval applies. For example, infantry has 2.0 attack vs naval units and that's the damage infantry will do to disembarking troops. Below is the unit card for infantry. Disembarking units will have their own unit card with their attack and defense values vs the various types.