Like I said. At first I am able to maneuver them, but soon, they get stuck somewhere in the water. Doesn't always happen, though. Help, anyone?

I forgot to mention: I am using Dr. Monalisa's UP 1.5.

Hi.
Those "big ships" like Titanic (1.5) or Anti-Missile ship can stuck sometimes at docks, but I never noticed any problem in the middle of the water.
Maybe you're enabling the Coordinated Attack accidentally? You can enable the Coordinated Attack by pressig ' (apostrophe) key, or CTRL + ' (EE2: AOS) or by pressing a button in the bottom-left corner, under mini-map. After that you can give commands to units, and when you press the Coordinated Attack hotkey (or button) again, all commands will be executed.

I noticed that it happens when I click a destination on the minimap, instead of on the main map. But thank you for the mention about coordinated attack, I didn't know that that was how it worked.