Lets say I have a submarine placed 1 sea zone away from a coast that has enemy fighters. Can those fighters attack my submarine, if so what defensive actions can my submarine take?
Yep. Whatever that country has in their bank is now yours.
A capitol fall is very hard to come back from because of that. The toppled country misses a turn of buying units (no $$$) while at the same time giving the other side the chance to purchase with their normal collection plus the plundered treasure.