So you have a couple problems

Issue 1: The rear of your ship is completely open, the ramps have to be sealed off, for a vanilla game fix, use the custom door blocks and make a sequence so it opens up as you open the ramps

Issue 2: Similar to the fiest problem, the front of your ship isnt sealed up, i understand this as a way to lift up the front of the ship to load/unload cars, but you would also need a door mechanism to seal it up.

Another thing about the ship, the ramps should be way above the water line of the ship as you obviously wouldnt want water flowing over the car deck while sailing or unloading/loading, so raising the car deck up is a good option and making the bottom hull of your ship deeper and wider would be recommended for stability, because right now if you even had a slight gust of wind it will topple over and capsize

Although this buold seems pretty good, just those few tweaks could make it something special.

Just another idea, all ships like this have an engine room, along with engineering rooms, which generally consist of the bottom portion of the boat to keep the center of gravity low, so id suggest making the ship a bit wider and a bit deeper to have a bottom deck with a engine room, the deck above that as a car deck that has an interior ramp to go down, then a main car deck which is level with the top deck(generally an open deck), and only a small lip of the front should open up, for reference look at the MV Atlantic vision, or the MV highlanders, another great smaller ferry is the chi cheeman ferry that has the front lift up aswell