There are 10 packages in each of the 8 levels. Most are easy to find and acquire but one or two in each level may require quite a few tries to land on it just right. Play a private multiplayer match so you can look for the packages without the hassle of AI finishing the match before you have found everything.
Note: If you get these online in a private match, the packages become unavailable to all players if the host gets it first that round. This only happens in the match when the host picks up the package. So have the host wait near the package and let all other players get it first then it will be safe to get in that game. Anytime a game is played after the host has it, the package will not glitch.
If you can't remember which level you are missing packages on, go to the Boat Selection screen and see if there are any locked boat skins that require all packages in a certain level to unlock and that will be the level you are missing. Every level has a boat skin related to that level's hidden packages.
View this video guide for all 8 levels.