"Eliminate the Guards" is an objective in The Tea Party
This is the time for you to start getting more used to using your assassin recruits in combat. It is better to save Chapheau for the second group, on the left, as they are much larger and the extra hand is useful.
The first group, on the right, has 6 Soldiers that you need to deal with. That makes it a very good brawl for you. Take them out and then head for the opposite side of the docks. Take a little time to loot the bodies to replenish any supplies you expended.
After you cross over you will see 9 Soldiers that you will need to deal with. This is a good time to call in Chapheau and then cut loose in the large brawl. Since there are a lot of them be on guard for them forming up a firing line. Either charge them or grab some hapless fool who is nearby to protect yourself. Remember that any you knock into the water are immediately out of the fight. From there it is time to get onto the boat itself and attend to everything there.