Ports come before crossings
Do not treat water like normal land. Secure a coastal tile, build or preserve a port route, and make sure your economy can support the naval push before sending a major army across.
A port also acts as deterrence. Even one credible naval lane can stop a stronger neighbor from assuming the ocean is free.
Transports need warship cover
Transport ships are vulnerable because they lose much of their defensive value while moving across water. If an enemy warship patrols the lane, a large ground army can disappear before landing.
Clear the sea lane first. Then land on a peninsula, a weak coast, or a tile where you can immediately build defense and expand inland.
Naval checklist before you cross
Run these five checks before committing an army to water. One. Is the coastline safe, with no enemy warship patrolling the lane you plan to use? Two. Does an ally already hold a port nearby that you can trade through or stage from? Three. Do you have the gold to replace lost ships, since a failed crossing costs both the transport and its troops?
Four. Is your warship-to-transport balance right, meaning at least one warship clearing the lane ahead of any troop-carrying transport? Five. Do you have a backup land route, so a contested sea does not leave your army stranded? If you cannot answer yes to most of these, wait or deter first instead of crossing.
Warship-first timing
Sequence matters as much as numbers. Send the warship into the lane first and let it establish control, then follow with the transport once the water is clear. Sending both at the same time means the transport is exposed during the seconds the warship is still fighting for the lane.
Against a defended coast, do not rush a lone transport in. Bring the warship to contest the water around the landing zone first, so the transport is not caught mid-crossing while the lane is still being fought over.
Beachhead checklist
Choose a landing area with low enemy troop density, no nearby port cluster, and a narrow front you can hold. After landing, build defense before chasing land.
If the enemy has missiles ready, do not stack every troop on one beach tile. Spread pressure across two landings or wait for a better moment.