The mission generator is showing incorrect match ups between underwater Alien deployments vs land terrain.
There is no underwater terrain and land terrain... there is only terrain.
In the battlescape screen, please note the white explosion, it is only intend to work in an underwater map.
The mission from your screenshot
is underwater, depth 1... thus it is using underwater explosion.
So, if you are using underwater deployment, should only use for underwater map. Not land maps.
There are no underwater maps and land maps... there are just maps.
Questions is can the current ruleset handle terrain restriction to certain deployment?
If not..
1) can underwater deployment be hidden to mission generator? But this will not solve problem for underwater terrain.
2) adding extra line to terrain and underwater alien deployment to indicate it is underwater.
3) Having restricted features in starting condition for terrains?
Yes, non-UFO deployments can be limited to certain terrains.
terrains List of string IDs of the terrains to choose from when generating the map. Not used for UFO land/crash sites, terrain comes from the Geoscape globe texture.
1/ no, mission generator is a debug tool... it's not something players should use... and modders should know which combinations make sense
2/ for what?
3/ no, terrain is literally unknown on the Geoscape... we only know the globe texture... and there is already a starting condition for that