Twin engine planes absolutely can cross oceans -- if you need three hours' flying distance in the worst case (ETOPS-180), then you can cross up to six hours of water between any two backup airports. North Atlantic routes have only a few hours of contiguous water between, say, Greenland and Iceland. Even the Pacific has enough small islands (e.g. Hawaii) to make something like this possible (shaded areas are outside 3-hour range):

http://www.gcmap.com/mapui?P=EWR+-+SYD&MS=wls&DU=mi&E=180