This is really easy. Got to descr_sm_faction. Then change the line

"prefers_naval_invasions no"

to

"prefers_naval_invasions yes"

And voila, the AI will go naval.

Tested this for the Celts and after taking London they launched a D-day on the continent. I don´t think all factions will go naval at once but some will given certain conditions.

Cheers

Boc