"Defend the people on the boats" is an objective in The Tea Party
Dump crates of tea into the water - 10
This is the trickiest of all the optional objectives to complete. Move to the crate you see ahead of you from your starting position. Grab and throw it over the boat's railing toward the bow. This helps increase the chances it will go overboard. Run from one crate to the next and lob them all over the railing in turn. You can easily lob 7 crates over before anyone is in danger with some practice.
After you have cleared the first wave of soldiers, run over to the second boat and lob all the crates you can get at on the bow of that ship into the water to finish this objective out.
Throw redcoats into the water - 3
Throwing the Soldiers into the water is a little tricky. You need to either use grabs and throw them in or you can brawl with them and knock them into the water. It takes a little effort either way. The earlier you do this, the easier it will get. It can be done simply over the course of the mission.
Perform musket air assassinations - 1
This is a somewhat tricky objective to pull off. Most of the soldiers carry muskets and there are racks of them found on the ship. To pull this one off you need to have a musket in hand, hop up onto the railing of the ship and then assassinate someone from behind. This can be done on the crates on the dock as well, but you are a higher risk of drifting out of the bounds of the memory.
If you are after a Full Synch, begin lobbing crates over the railing immediately. After that you need to watch the mini-map to know when enemies are coming in and on which boat. Follow the suggestions above to get the conditions then just focus on keeping everyone alive. Be swift while fighting as the Red Coats can do a lot of damage to your 2 allies without much lapsed.