The TRIREME is the first ship, an oared galley that carries 2 land units.
It must hug the COAST: alone in the open sea at end of turn, it may be LOST.
Requires MAP MAKING. Needs COPPER: +50% shields without. Made obsolete by NAVIGATION.
The risk is real; end each turn within sight of land.
THE LIGHTHOUSE grants an extra move to your ships, and quiets the danger of the deep. Something in that deep may notice.