What do these D’s mean

Right.

Firstly you need to kill the goblins. You can't finish anything until that is done - As others have suggested marksdwarfs are the way to get this done. Even an untrained marksdwarf may hit something sometimes so make a squad and just tell them to station on the floor above. They will fire through the holes and eventually kill all the goblins.

Once they are dead, you can build your hatch and be happy in the fact the goblins are now doomed.

Personally, i'd make a ramp and have the hatch a floor higher, so they drop 2z levels into the water - the first drop will stun them, and you'll be able to fill it to 7/7 and make sure they drown without them climbing back out :)

Red is the location of creatures and monsters, the red meaning "potentially dwarf-scaring or -mauling". Blue is the location of your dwarfs.

The colour ignores z-levels of air, so if there are fliers you'll have to hunt through the elevations to find out what the map is indicating. Also note that there is no consistent size relationship between you normal view and the map, so you might be looking at multiple map-squares or just part of one, depending on your screen size and the size of the embark area.