Shift has the problem of bringing up windows sticky keys dialog box if you tap it too fast. Spacebar was my instinct for fire key as well. For map it would be 'M'.

Whichever you choose, please consider displaying the controls inside the game, at least during the setup screen, don't make the user search for controls in the wiki. :)

Also the hp bar below the minimap is a bit weird, would expect it the same as enemy ships above my ship, so i can see if i'm winning or losing a cannon exchange like in a rts game. Or at least someplace more convenient to quickly glance at nearer the center of the screen.

Very fun game by the way, well done! Scratches the nostalgia itch of 'Corsairs Gold' for me.

Space was definitely what I pressed first to fire. Then enter. Then shift. Later I realized the menu has the key bindings in it.

Tab and M for map, WASD and arrows for controls, space to fire seems like a good compromise

I also tried using the space bar to fire, it took a bit of digging to realize it was the shift key, but space bar felt more natural for me.

My first instinct was space to fire FWIW (Also its giving me Age of Empire feels here!)

Space for fire, I kept hitting it reflexively.

Space to fire would be my preference as well.

Shift doesn't make sense as a main key almost at all tbh.

Space to fire makes more sense than shift to fire

Edit: Shift is a modifier key. Hold shift to go from lowercase -> capitals. In FPS games shift is walking -> running. In RTS games shift is "select one of these units -> select all of these units"

I know your game doesn't have to follow these rules, you can do whatever you want. However it's worth thinking about what "language" each key on the keyboard is speaking when you want to use it.