True. Unless you are vikings, you couldn't land everywhere. Sadly BM doesn't have nice sand beaches!
Maybe make it impossible to land in forest regions?
Really, landing was not hard, you only need a little beach. The only thing could do impossible to land would be an army... and even then, you can sail some hours more and land far away... I think they would be no restrictions on where land (aside the existence of enemy armies)
About the speed of sea travel? Fast. At least faster than by foot.
A Question... The sea travel will be 'oceanic' or all the sea-regions will be coastal? (I, personally, like the last option better!)
mmm...
Another thing. If you can embark only on a harbour... this mean an army who land on a enemy region/realm will be 'trapped'! If they can't take a city or a town region they will never be able to 'come back home'?
IMHO in a harbour would embark all kind of units, and in a non-harbour region only infantry? or small infantry units? ...or at least nobles without units.
