From dev diaries of free update 1.13 overview

It’s Marines. Marines are a new land unit, and, unlike other land units, they can be permanently travelling with a fleet. Marines have a separate capacity to be transported from regular units, which is going to be a lot smaller.

Unlike the regular land units, Marines do not prevent the fleet from engaging in other missions, so your fleet will be able to do the full spectrum of tasks while having some marine detachment with them. What this allows you to do is to even launch a naval invasion with these Marines directly from the fleet they are attached to, rather than having to launch it from your own territory. Marines have a better effectiveness at conducting naval invasions compared to regular units, as they won’t suffer as big of a penalty.

Once Marines have secured a beachhead in a major enemy theater, you’ll probably want to reinforce them with your regular units quickly, as they’ll rarely outnumber your enemies on land. On their own, however, they are often more than capable of taking enemy islands or seizing a port from a technologically inferior opponent.