Good evening. I found the home system of the Vodyanis and I noticed they have a ship called Ark which has a lot of HP , decent offense and great defense at least for the beginning. Please  give me some tips how to fight this ship and what are its strengths and weaknesses. I only explored 20% of the galaxy and I'm a novice in the game.

Vodyani Arks are their colonies. They're scary in the early game, but if the Vodyani uses them to attack they're using what could be a colony to do it. You don't have to worry about the AI attacking with an Ark and the AI is fairly incompetant with the Vodyani in general. Killing a lone Ark is fairly easy; a half-dozen attackers with missiles and shields are a credible threat to a lone Ark, a full dozen will take it down easily. It's when an Ark is backed up by a fleet in the early game that it becomes something to really worry about, as with equal CP, tech levels, and only small ships on both sides, the fleet with the Ark will almost assuredly win. Again, the AI doesn't do this; unless you're doing multiplayer you will never have to worry about an Ark rush. If you have to fight an Ark rush, spam missiles with attackers and have multiple fleets, or rush Hunter hulls.

If the Vodyani anchors their Ark, letting it act as a colony, and has a meditation module equipped, you cannot directly attack the Ark and must invade it like you would a regular colony. Typical invasion tactics will work, except you have to invest much more into the attack, as the Ark will deploy its manpower into the ground battle every round. Siege it down first, and invade with a fleet equipped with manpower deployment cap increasing modules, which you can find in the military tech tree.

Thank you ANGRYABOUTELVES!

Hi ! Sorry for my english, but i need advice about Vodyanis ark. I am in war with this fraction. Ark ship came in free system in my area, in one time colonized and lock it. And i cant do anything with this. I can't destroy ship because it is untouchable. I can't win ground fight cause somehow it have endless ground troops and i have limited troops in my ships. Enchanced infantry 5\5 tanks 4\5 aviation 4\5. Occupation shows 0% defense. Is it ok ?

You'll have to siege the system down to 0, then bring ships with Manpower modules to increase your Deployment limit until you can overwhelm the fresh Manpower that the ship will drop combined with the Draft mechanic

This gets easier later in the game when you've increased your Empire Deployment limit.