Assign Franklin as overwatch at the lot. Covering Michael, you can actually use just a single ammo to take down the couple of enemies. After you have cleared a path for Michael, switch to him and rig the first explosive. Change back to Franklin asap and move ahead of Michael. Kill all the enemies in the gangway as fast as possible and watch out the stern, as there are loads of enemies here. The bridge and upper decks are filled with baddies as well.
Once Michael enters, clear the guards and cover Michael's escape going to the bow. Also, make sure you clear out the stern and stairs as well. When Michael goes into the water, trigger the explosives. Look out for Michael using the red indicator afterwards. Have Trevor enter inside and grab the weapon, then return it to the port.
