Yeah you could buy it as addons, but now you have the possibility to even get a higher stat for heavier terrains.
And these are already included in the costs, you can substract them and you would get pretty cheap troops.
But in heavier terrains you do not use the Sea Lions, because they have no good Weapons for short LOS, which you have in heavier terrains.
In heavier Terrains you need SMG, Shotguns, HMG, Flamer or real (with two wounds) CC troops.
Thats the Problem of the ACG, too. It were such a great unit for dense jungle fight if they would have keept their shotguns from first edition.
Assault Rifles for heavy enviroment, argghhh. I always can trouble over this topic.
For me its really a pitty that the Sea Lions with cool minis and great fluff is nearly useless.
Maybe except on a two level field, but I think most players don't have a two level field.