Like Wittmann said.
You can place any naval unit in a SZ occupied by the enemy. Nothing happens at that time.
Comes the enemies’ turn again he/she must -all in the Combat Move phase- fight those units, move away to initiate combat elsewhere or move away without initiating combat anywhere.
Note that your enemy doesnt have to do anything if you placed only submarines/transports, as they may be ignored.
Also note that your enemy is not allowed to load land units into his/her transports in such sea zones (never in ‘hostile’ SZ anyway). But they can move out, pick up land units elsewhere and return to the SZ during the same Combat Move, to initiate battle (together with other naval units that might have been moved in).