Yeah, those one tile islands are a pain untill you get marines. And even then, carefull. By the time you have a good sized marine force, chances are that island has a few Mech Inf. defenders.
I suggest you heavily, and I mean really heavily, bomb it before you attack. You could loose boatload after boatload of marines on even only one fortified Mech Inf. defender.
What I tend to do however is not bother with too many marines since they're expensive to build and not worth it for a few little islands. Instead build bombers, lots of them. in PTW/C3C they have leathal bombard. So just bombard the city till all defenders are dead. Then all it takes is one marine to land and take it.