I like both, only the disadvantage of Islands is that you can quickly go to another territory on the other side of the map which means you can spread faster than with boats. I think a Island connections should be limited to 2 or 3 territories to avoid quickly expanding, which is not fair compared with expanding by land. An example is the Big Africa map https://www.warlight.net/Map/10589-Africa-Big
Boats do represent real life situation. People use boats to travel over seas all the time. Only to get a bonus on a sea/ocean is not realistic because you cannot build an army on sea if you don't have land.