You can do the same.
Sadly its just a result of how CA "fixed" AI by changing water to be viewed as land with movement debuff.
Now what happens, every army that disembarks in port, both player and AI, will get massive movement bonus. This is the reason why transport fleet are that slow, as their movement is translated into that bonus. For example when we made transports to be as fast as navies, when you dismebarked in Beneventum, you could go next turn up to Alps.