Some players have decided on an informal rule to make it more difficult to attack across the sea. Players may decide that the defender receives +1 on his highest defending die. For example, a defending roll of 5 and 4 would become 6 and 4.
An alternative to this is to make it more costly to attack across the sea. Players may agree that for every time an attacker rolls a 1, an additional unit is lost "to the sea". This rule can make an amphibious assault become a great triumph or a costly disaster.