Attackers roll one die fewer across a sea lane, for five rounds

For the first five rounds, an attacker crossing a sea lane rolls one die fewer than usual — two instead of three, or one instead of two — but never fewer than one. Everything else about the roll is unchanged, and from round six it goes back to normal. The penalty falls on the ATTACKER rather than adding a die to the defender, so no roll ever compares more pairs than the rules define. Sea lanes are the chokepoints the whole map turns on, and taxing the early amphibious rush gives everyone a few rounds to build something first.