[BadgerDog]

You can buy tonics from Gypsies on almost every island. If you sail to Tortuga and follow the path to the right of the jail, a you should find one. Just press shift to talk to them and choose buy.

As for boarding ships, you need to find a flagships, a ship that has a flag above its name. Attack it, and get it down to 0 health points. Then, green circles should appear on one side of the ship. Get up close and get to a cannon and press 8 on your keyboard. Aim the hooks at the circles until you get the prompt to board.

And as for getting a flagship, sorry, you can't. The ship will sink as you re-board your own ship.