This is a cute one. I like that there's difficulties especially as a way of having a kind of tutorial cos at first I thoguth we would be looking for sailors who had gone overboard! So yeah easy just spells it out which is a good way to understand the game, then medium is a game of memory and finally hard is for people who actually know semaphor! Love that it's in there.
This is a really cool idea - though there are a couple things that hamper it. Firstly I did get a lot of the same messages so once I saw the first word or two it was easy to go and type the whole message and move on. Second there's a glitch on medium and easy difficulties where you can just exit the telescope and still receive the letter prompts so you can just got to the machine and start typing.
It might be nice to have a mode between medium and hard where the ships don't pause and you have to focus on the message as they keep sailing. Then it's simultaneously a game of aiming AND memory - I think that would be a nice sweet spot. Though that might require shorter messages to work.