Indeed bridges are for rivers only, not over ocean. This is very realistic. Bridges in real life that are not over rivers are over narrow bodies of water. In civ2 an ocean tile is a rather large body of water. Look at the large map of earth: Persian Gulf is one tile wide in many spots, so is the English channel. If two bodies of land share a corner and are otherwise surrounded by water you can go from one to the other without a bridge and ships can go across as well.
In Civ1, you could indeed build roads and railroads over water as Sir Lancelot mentioned. They would give you the terrain benefit (extra resources) but no unit could step on them. That was an oversight and has been corrected in Civ2.