First I tried using the Task 'Move to Hex' and its settings 'Target Hex and Disembark Hex.
The units will simply not follow orders... I've even had units moving sideways along the coast instead of debarking on the coast next to the units starting point.
To cut a story short (maybe too late for that...
I created a number of deploy units effect at various turns. Please see screenshot where the seaborne units will deploy at turn 1-2-3-4.
Then I undeploy all units. I don't think you even need to do that if you want to keep the unit overview. The units should just be redeployed.
A nice side effect is that you can assign commanders to the undeployed units (this is impossible with spawned units).

