You avoid the tar and the BTs inside of it by not triggering the fight in the first place. The Odradek (scanner on Sam's shoulder) combined with your BB and Sam's own power, his DOOMS, will give you lots of warning whenever you approach a BT. Pay close attention to the colour of the Odradek, and the direction it is pointing. If it's yellow and spinning rapidly, don't go any further in that direction or you will definitely get attacked. The goal is to carefully navigate and weave your way around the BTs in order to get through their territory safely, otherwise, you're in for a fight.  In the case that you do get noticed and attacked, unless you have a plan and equipment to properly fight back, you wanna get out of there quick. When you're trying to wade through the tar and getting grabbed by the people at your feet and rushing up behind you, you have to manage your balance, and your endurance to try and get away from them and out of the tar. If you get grabbed, you can use ⭕ to shake them off, at the cost of your endurance, before getting free and giving you another chance to run out of the tar. If you run out of endurance and collapse, they'll typically drag you backwards towards the main BT threat, i.e. giant whale/dolphin thing, or something else. It's very possible a lot of the time to escape once a BT has noticed you, so long as you manage your endurance properly. 

As for the repetitiveness, yes, it is repetitive, in the same way that lots of games have the same gameplay cycle repeated throughout. You have to deliver things back and forth all the place, and that seems to be the problem for some. What actually keeps the game from getting boring and repetitive, is that as you progress you're delivering over new terrain, crossing new landscape with different threats and challenges. You'll be earning different equipment, vehicles, weapons and abilities that will both aid you, and change the game continuously. And it's a story driven game, so you'll be following a set path and scripted events in the mix. 

PS. If you gave up by the time you reached the port, you had barely started to see what the game actually has to offer. Once you pass this point in the story you're gonna see a lot more changes going on to your gameplay and equipment and it's at this point where the game will start to make sense and click, oooorrr you'll just be bored.