Well, there is a body of water called the Gulf of Finland between the two. It’s about 100km by Ferry from Helsinki to Tallinn. Maybe check on the ferry schedules to see how many per day. Looks to be doable but not as a day trip.
I don’t think it’ll be easy to do that because the ferry ride is actually kind of long. I know some people do it, but it’s better to spend at least one night in either Estonia or Helsinki. I did this trip once and staying in Estonia proved necessary. It’s much cheaper to stay in Estonia than in Helsinki though.
I’ve not been to Helsinki or Tallinn, but I have been to Stockholm, which allegedly is like the same as Helsinki except it is considerably larger. Everything closed at like 5pm and it turned dark at like 11pm in June.
Wife & I did an extensive trip thru Scandinavia Summer 2019. From Helsinki we took the train to St. Petersburg, with VISA processing done while you’re sitting on the train after crossing the land border. Super easy and faster than doing airports. Read stories of long waits at St. Pete airport upon arrival for immigration, so the train was the right call.
After 5 days in St. Pete, did the train back to Helsinki and with an approx. 2-hour window caught a tram outside the rail station to one of the ferry terminals for Tallin. Made the connection in plenty of time. Ferry was big, comfortable and lively. Lots of Fins heading to Tallin for the cheap booze. Lots of duty free shops on the ferry. many people simply doing ferry “booze runs” and returning on the next ferry back to Helsinki.
Highly recommend a couple days/nites in Tallin. Certainly one of our favorite stops on our trip.
Finally, Helsinki is nothing like Stockholm. Surprised a poster said that.
If I recall there are 2 (3?) ferry lines plying the Helsinki/Tallin route. Do your research on times and ferry terminal locations. Buying a r/t on one line does not allow you to use the other line if you’re stuck after missing your original ferry. You’d need to buy new tix.