I have played this scenario a fair bit and still notice the same problem... I am doing most of the fighting. Even when USA is at war with Germany for five years it never send a unit. Japan and Russia at war for three years exchange one city. In my war with Gemany, Italy etc... Not once did I feel threatened as the English.
Not sure why this is happening or what can be done.
Anyone else have a "different" experience?
Is it the map?
Is it the number of units and "era" the scenario starts in?
Do the other scenarios have a more robust AI combat dynamic?
With regards to the sea/oceans I usually encounter the same problem in many scenarios as the AI rarely loads units on galleons/transport etc to invade and attack.
Unfortunately even installing the Better AI Mod, didnt help much.
If there is an ocean in between, even if a narrow one, with just a single sea square (like Gibraltar for Spain/Morocco), the civs tend to treat it as a major obstacle.
The AI is thick.
What I noticed is that to trigger "invasions" via sea, it helps if you actually place an aggressor unit into the enemy's mainland.
The unit gets annihilated but the aggressor AI "acknowledges" the existence of those lands and
might consider attacking.
I wont lie though, it happens rarely.
On my mod, 1205 AD GEM, England very rarely gets attacked even though it starts at war with France.
However if I place enough french units in England so that they manage to take over one english city, then the AI start sending units from the French mainland into England.