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.
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.