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As others mention, show the keys in the interface. It took me a while to discover accidentally, by pressing space, that I can trim the sails. You could make a graphical guide, showing a ship with arrows around, some arrows attached to the hull with keys responsible for turning the ship, some arrows attached to sails with keys responsible for trimming them. Yet another key by the cannons fire to show which key... umm... fires the cannons...

The whole thing should ideally have a quick modal with hotkeys when you first open it (and a small button to bring it up again if needed).

Modals are the worst ux in games.

If it's only a few buttons anyway you can just show them somewhere as hints while the user isn't giving input or until they used then.

Eg just show a "m for Map\nspace to fire" until the user used each, hiding them each when the m/space is used respectively for that session (session storage so it shows up again if the user comes back later)

Space to me means fire.

You can show the mini-map in a corner, right? Why do you need to toggle it, Slither has it shown at all times.

I agree minimap on corner is ideal, but it does make it easier in a way