Different transports, which are what defines strength of embarked units.

DNLH wrote:

All embarked units should have CS of naval transport the Empire has access to, though, AFAIK, various bonuses still apply. Naval transport units are:

Transport Galleys - the most basic one

Langskip - available only to Norsemen in Medieval

Caravel - unlocked by researching Three-Masted Ship in Early Modern

Fluyt - unique version of Caravel available to Dutch, more strength and faster movement, moves even faster if starts in own territory

Landing Craft - available to everyone upon researching Amphibious Warfare

The CS jump you mention is most likely difference between Transport Galley and Caravel/Fluyt.

EDIT: Once again, I sure wish g2g would warn about new posts appearing in the thread when you were busy writing down your answer.

Thanks for the info - in fact, it was the Dutch who had the much higher STR

Has there been some change in embarked unit CS?

I don't have screen caps, but I swear that previously the CS of embarked units was just as described here - i.e., the CS of the latest naval transport is used, but currently it seems that the CS is actually the CS of the unit, not the naval transport. My recent experience was fighting with my naval units against embarked independent people units, and the combat preview showed the CS of my Quadrireme or Cog vs the CS of the Knight that was embarked. I had checked that the naval transport of the Knight was the basic Transport Galley, and that's the CS I expected to see.