In the options change the fast-forward speed to maximum (all the way to the right). Also have auto-docking enabled.
I play on Cyclone Island map. I also hold my phone in portrait mode as I find it easier (you can play with one hand). Start the game and hit the fast-forward button in the top right (or bottom right if holding in portrait). I find on this map you have a lot of room to manoeuvre the boats and a lot of the boats head straight for the docks.
The game-play itself will be quite manic at first until you get used to the speed. The key point is to make sure you move the unloaded boats out ASAP. I usually get overwhelmed after a minute and have to re-start which only takes a couple of seconds.
On average I'm getting around 30 boats per minute but, depending on your skill, you should get 25-35 per minute. This means to get the 50,000 is going to take you around 25-35 hours when you consider completing the other achievements too.
You can check your progress under Leader boards - Cargo Received from the menu. I hope this helps and good luck with the grind!
UPDATE Sept 2020
The saved data is device based, not cloud. If you lose the app/phone, you'll have to start over again.
I restarted on a phone with a larger screen, which made the game easier to play. As a result, it meant I could finish 1,000 boats in just under 30 minutes. Use a larger screen and you'll get this finished quicker.
Also, the leader boards no longer seem to load, unsurprising given the age of the game, so keep your own tally,