I really do feel like Marines were meant to be stacked. If Woelf’s lurking maybe he can correct me here, but as the only ability that can stack in the game I wonder if Wizkids meant for people to play 2-3 Marines per fleet, like an army to compliment your navy. But their point cost would make even less sense in that case.
You can absolutely do that, it’s just incredibly impractical. No sense in paying 12-18 points for 2-3 extra cannons when you can get a good gunship with crew for the same cost.