The minigun is located in- "entity/weapons/MounterWeaponMG"
The tripod for it is located in-"Brush/minigun/tripod_with_ammo & tripod_with_ammo_a"
I dont snap with miniguns(although im sure you could), i place the tripod then place the MG. Make sure you test it out because you'll more than likely have to tweak it a few times to get it at the right height.
Also keep in mind you can edit many parameters such as the angles youll be able to cover/sweep with the weapon ect.
From what i hear your probably looking at the "brush" directory, you say there is a boat model with and without MMG.The only models without MMG in sandbox are brushes.
All you have to do is place the boat entity (Entity/BoatPatrol or Boat) where you want in the water and when you play in engine or after exporting everything will be GTG, MG is pre-mounted