When building the patrolboat the patrolboat looks okay. But when I enter the boat to drive it around, it gets something like a ghostly outline. Kinda like the graphic of the patrol boat is painted again half a tile above the boat, but this time half transparent.

hmmm... I don't have this problem... try deleting the Battleship graphics package and downloading it again

Test with fresh download, fresh map still persists only mod is base and battleship (and its dependencies):
after building
https://cdn.discordapp.com/attachments/231518135231053825/1482663960644943902/image.png?ex=69b7c5ea&is=69b6746a&hm=f296ce1f756148c9d88fdc8e1c039de80a7a64c1705ad32681a5a41e19c4b790&

after entering the patrol boat
https://cdn.discordapp.com/attachments/231518135231053825/1482663961135681536/image.png?ex=69b7c5ea&is=69b6746a&hm=9d3494c60996872a69b7c3446b5239635ace3adcf3c33da485b3832a33b3616a&

try 0.3.28