How to properly perform sea invasions?

It's a Long story.....

I was playing as Greece in world at war map, when by day three I saw Libya literally having more than 300 VC in day 3...I was certain that he was cheating or at least was an ultra gold abuser...so I made an unofficial alliance with Italy and Belgian Congo and brought in my coalition members (or what was left of them) Persia and Spain (all those where bordering him... that's how massive he got..)....

I have played 5 games and watched several YouTube tutorials...so at least I am experienced enough to notice cheating...but not enough to have proper knowledge in sea invasions...which are things i haven't done before that...

Well yes.... I grouped up a lot of subs...a couple crusiers and destroyers and a battleship to escort my 25 land units straight into his capital....after all i was supposed to catch him off guard in his own core provinces...i would even talk to him giving him intel about another coalition in southern africa conspiring against him (which was true as many would talk to me about how dangerous Libya is) so he would trust me and lower his guard....

Now yes I made a mistake not suspecting he would have so many spies as to know everything planned from the start and destroy my army right before finishing disembarking with 10 artillery units....

It would have worked (if I even just took Tripoli I would steal 500k cash...enough to build twice as big an army I would say!) If only the ships escorted the transports...I thought they would do that in disembarking just like in traveling all the way to the Libyan shores....

Why didn't they do it?

And how can I do it if possible?

Sry for the big post I thought you can also give me diplomatic advice as well about the whole situation...